[Reader][Win7] 21208 update breaks Acrobat Reader, won't start, missing dll

Mar 13:  Bobthecat woke up from a nap on a warm windowsill to find something new in Adobe reader:

1. Checking Programs and Features, Adobe Reader on 3/10/2026 updated itself on BTC’s computers to Version 25.001.21288.  It seems to run, BUT……the new version may produce the hateful AdobeCollabSync.exe error message.
2. You must check the Adobe program directory Reader to see if the most current version of adobecollabsync.exe is still disabled.  The date of this file on all of BTC’s computers is 3/8/2026.  This is the one that must be disabled.  If it were disabled for 21265 it is probably still so, so that 21288 should still work without the error message.  If the 3/8/2026 file is appearing for the first time, rename it to disable it.  Also disable firsttrustnotification.exe as a precaution.
3. Installing Version 21288 is much easier than 21265 because its installation file can be downloaded from https://get.adobe.com/reader/enterprise .  There is no longer a need for the msp update file. 
4. Reader may or may not run correctly once newly installed.  You may get the AdobeCollabSync error message.  If so, simply disable the latest adobecollabsync.exe (currently 3/8/2026) in the Reader folder. You should delete older versions of this file (BTC had 3 older versions on one computer). Reader should then run correctly.  It did for BTC once he disabled this file when Reader was installed on a computer that did not already have a working version of 21265.

BTC, being a simple cat, does not understand why Adobe persists in including AdobeCollabSync in the 32-bit versions for Windows 7, knowing full well it will produce the error message.   Disabling adobecollabsync.exe is a band-aid, at best, because every time a new version of this file comes along (in a program update) it will have to be disabled.  Many users, and BTC’s wife is one, would have no clue how to do this.  The very least Adobe could do would be to put in “Preferences” the option to turn off (bypass) Collaborative Synchronization to prevent its trying to load. 

Meow

Probably the final update - March 10

Here is a summary of how Bobthecat got a working (paws crossed) version of Adobe 32-bit Reader on his 64-bit Windows 7 computer.  It does not require downloading the missing api-ms-win-core-winrt-l1-1-0.dll file.  But, before jumping into this you might want to read it carefully and look at the FINAL comments.

STEP ONE - DOWNLOAD 3 FILES:  

 Download  the following three files and put them in a folder with a name like "Fix 32-bit Adobe Reader"

FILE 1:   Adobe Cleaner Tool, used to clean up traces of any Adobe version AFTER it has been uninstalled using Microsoft "Control Panel:  Programs and Features:  Uninstall Adobe Reader."  DON'T UNINSTALL ANYTHING YET - JUST DOWNLOAD THE FILE

Site:    https://www.adobe.com/devnet-docs/acrobatetk/tools/Labs/cleaner.html

Select:  “ Feb 9, 2021:  64-bit installer  All tracks.” 

Note:  BTC was unsure if "64-bit installer" referred to the Operating System or to the Adobe Version.  Since his Win 7 is 64-bit, this is the one he chose.  It seemed to work.

FILE 2:   Version 25.001.21223 32-bit (starting-point file):

Site:   https://get.adobe.com/reader/enterprise  (see post by HelpfulJohn).

There is only version listed:   Reader 2025.001.21223,  English UK for Windows 32-bit

The file that downloads is:   AcroRdrDC2500121223_en_US.exe

FILE 3:  Version 25.001.21265 32-bit update (Win 7):

Site:  https://www.adobe.com/devnetdocs/acrobatetk/tools/ReleaseNotesDC/continuous/dccontinuousmar2026.html#dccontinuousmartwentytwentysix

File (2nd in 32-bit list):   AcroRdrDCUpd2500121265.msp

STEP TWO - INSTALL ADOBE READER  Ver 25.001.21265 (round-about way)

Open Microsoft "Control Panel:  Programs and Features" and see what (if any) Adobe Reader Version is currently installed.  If it is Version 25.001.21223 it may be the one downloaded by Adobe, and is probably okay.  But if it is any other version (or you just want a clean start), uninstall it.

STEP THREE

After Uninstall, run the Adobe Cleaner Tool to remove any traces left over by Uninstall.

STEP FOUR

Use File 2 to install Version 25.001.21223

Look in "Control Panel:  Programs and Features" to verify that 21223 is the installed version.

Once installed Adobe Reader WILL NOT RUN .  You will get the error message "Missing api-ms-win-core-winrt-l1-1-0.dll".  IT IS NOT NECESSARY TO DOWNLOAD THIS MISSING FILE AND PUT IT IN C:\Windows\SysWOW64.  The next step will fix this problelm.

STEP FIVE

Use File 3 to update Reader to Version 25.001.21265 (32-bit)  -  note, BTC’s Windows 10 computer has Version 25.001.21265 (64-bit).  They are NOT the same.

Look in "Programs and Features" to verify that 21265 (32-bit)  is the installed version.

Once this update is made, the "missing api...dll" should be fixed and Adobe Reader should run.  BUT you will probably get the error pop-up titled "AdobeCollabSync.exe" in the header.  Exit Adobe Reader.

STEP SIX

Click on the Desktop "Computer" icon and "Local Disk (C:)" see the contents of  C:\ .  Click on "Program Files(x86)"  then "Adobe" then "Acrobat Reader DC" then "Reader".  Scroll down to file  AdobeCollabSync.exe  and rename it AdobeCollabSyncOLD.exe.  This should disable it.

NOTE 1:  Adobe seems to insist in forcing this file on the users.  BTC found three copies in the Reader folder, dated  11/17/2025, 2/25/2026, and 3/8/2026.  In Nov 2025 BTC renamed the 11/17/2025 version xAdobeCollabSync.exe.  But the other two files (with the same name) were there on 3/10/2026.  He renamed the 2/25/2026 version xxAdobeCollabSync.exe and the 3/8/2026 AdobeCollabSyncOLD.exe.  The OLD keeps the last file in its original position, so it will be easy to see if another one comes along - which should cause the error box to occur again.

Continue down to FullTrustNotifier.exe  and rename it FullTrustNotifierOLD.exe to disable it.

NOTE 2:  BTC found two copies of FullTrustNotifier.exe, dated 2/25/2026 and 3/8/2026.  He renamed them xFullTrustNotifier.exe and FullTrustNotifierOLD.exe, respectively.

BTC is curious how Windows allows/shows multiple files with the same name.  A quick Google search says it can't be done.....but it was.

FINAL COMMENT

As of 3/10/2026 Adobe Reader seems to be working for BTC on three computers.  But he has no confidence in it, especially since files like AdobeCollabSync.exe and FullTrustNotifier.exe keep re-appearing.  He will not go through these steps with his other computers.

For now, your Cat has reached the end of the trail.  He is a simple Cat, not Don Quixote.  He does not intend to waste more time with Adobe Reader and will use SumatraPDF (although a friend recommended Foxit) as his default reader.  He does not need or want Collaboration, AI and all the other stuff.  He will occasionally check Adobe Reader to see if these fixes are stable.  He will bet a bowl of cream that they are not.  Meow.

Update - March 9

For 32-bit Version 25.001.21265 (Windows 7, 32-bit)

ONE:   Yesterday:  In  C:\ProgramFiles(x86)\Adobe\AcrobatReaderDC\Reader,  BTC renamed AdobeCollabSync.exe to AdobeCollabSyncOLD.exe   and   FullTrustNotifier.exe to FullTrustNotifierOLD.exe.  As of today, Reader still works without the hateful AdobeCollabSync error message.  Reader now seems to open faster, without the Adobe Collaboration Synchronizer. 

TWO:  BTC deleted the registry entry that was supposed to do the same thing because yesterday (with the registry entry but not renaming the two files as in ONE), the hateful message  came up again several times.   At about the same time the last message came up the computer crashed (blue screen).  May be a coincidence, but BTC does not believe in coincidences.

As an aside, the 64-bit Reader 25.001.21223 on BTC’s Windows 10 (64-bit) computer is different. 

AdobeCollabSync.exe  is in C:\ProgramFiles\Adobe\AcrobatDC\Acrobat .  BTC appears to have disabled it (by renaming) in 11/2025 but Adobe reinstalled it 2/2026 (probably with the 21223 update) and it is active again. 

FullTrustNotification.exe is in C:\ProgramFiles\Adobe\AcrobatDC\Acrobat\RDCNotificationClient.  BTC did not disable it in the past.  So, disabling AdobeCollabSync.exe must have been all that was necessary.
There are some registry key changes recommended on some sites, but BTC is wary of those.

Since Adobe Collaborative Synchroncizer is so intrusive and odious (may users report it locks PDF files so they cannot be moved or copied) BTC will see how to disable it in Windows 10, starting with disabling AdobeCollabSync.exe (again).

 BTC will continue testing and will report.  But, if the error message comes up again (in Win 7 or 10), then it is Bye-Bye-Adobe, and Hello-Sumatra - for good.  Your Cat has spent too much time going down rabbit holes.

BTC remembers many, many years Adobe Reader (free version) was “every-man’s reader”.  It was simple to install and use.  It was a "hook" to get the user to buy the full Acrobat.  But now it is bloated and (for many users) unworkable.  Perhaps Adobe should issue “Reader Lite”, without the baggage, for users who do not want “collaboration”, the cloud, and AI.

Update - March 8

BTC got a chicken bone in his throat this afternoon.  After making the registry change that he found (but has not yet posted) Adobe opened for several days consistently without the hateful AdobeCollabSync.exe  error message pop-up.

But today, it has begun again (yes, the registry change is still there).  So BTC has implemented the easier change which is to simply disable (by renaming)  two Adobe system files that “activate” the Adobe Collaborative System.  This was recommended by users on on other Adobe sites.

The two files are AdobeCollabSync.exe and FullTrustNotifier.exe (be certain that Windows Explorer is set to "show file-name extensions”).  For the 32-bit Version 25.001.21265, start in Drive C:\ and follow the path (the folder names are those shown on Bobthecat's computer - your may vary slightle):

            Drive C:\ ProgramFiles(x86)\Adobe\AcrobatReaderDC\Reader     [be certain to use (x86)]

Within the \Reader folder, scroll down to find files:  AdobeCollabSync.exe                                                                                                               FullTrustNotifier.exe

Change the file names to something like:                  AdobeCollabSyncOLD.exe                                                                                                         FullTrustNotifierOLD.exe

This disables the files because Adobe won't find the files it is expecting.  Adding "OLD" at the end of the file name will keep these files in the original positions in the \Reader folder, so that if a later Adobe updated puts in new versions they will be easy to spot.  By not simply deleting the two original files, you can undo the change by restoring them to their original names.         

BTC will keep you posted.  

I uninstalled Version:  25.001.21265 because it had TWO errors!

I then installed version: AcroRdrDC2500121223_en_US and now this version will NOT open and has the same damn error: api-ms-win-core-winrt-I1-1-0.dll is missing from your computer

So WTF do I do now?

People keep saying download the patch but WHICH one exactly?
All of the links have multiple rows of patches!

Can someone PLEASE answer me and include a link to the correct patch?

Thanks!!!

Update #2 - 6 Mar:   At the risk of being overly optimistic, Bobthecat (after a lot of catnip) thinks he has found the solution for the intrusive popup he posted earlier.  The key is “AdobeCollabSync.exe” in the header.  Paraphrasing a Google search result:   “AdobeCollabSync (Adobe Collaboration Synchronizer) is an Adobe background process that synchronizes shared PDF reviews, cloud documents, and comments across Adobe apps like Acrobat and Creative Cloud.  It acts as a bridge for cloud-based collaboration features, allowing real-time syncing of documents.  It often causes performance issues, such as high CPU usage and locked files, where PDFs cannot be edited or moved (see Adobe’s community forum).  IT IS NOT ESSENTIAL FOR USING ADOBE READER FOR BASIC PDF VIEWING AND PRINTING”  (italicized emphases by Bobthecat).

There are two methods that seem to be the most recommended for disabling AdobeCollabSync.  1)  Disabling two files (deleting or renaming - BTC prefers renaming since it can easily be un-done),  2)  making a single entry in the registry (this is daunting for users not familiar with mucking in the registry).  BTC has done Method 2 (registry entry), because it might be less affected by program updates that Method 1.  IT HAS SEEMED TO WORK SO FAR - but BTC has had to eat those words (without the benefit of catnip) to meow loudly just yet.  He has not tried Method 1, but it seems easy to do and has been said to work without having to get into the registry (intimidating and risky).  Doing both methods are the “belt and suspenders” approach. BUT, there is a strong possibility that Method 1 and maybe Method 2 might be overwritten the next time Adobe pushes down another involuntary update. 

Your BTCat will try to post both Methods (with detailed instructions) tomorrow, 7 Mar, if the solution still works as it does today.

Update 6 Mar:    In yesterday’s post I described updating two computers to Version 21265.  This morning, on both computers, Adobe Reader is not working properly when opening a pdf with Adobe -  gives error message:

Hitting OK three times makes message go away.  Reader seems to work, but there is no way of knowing what components are not working.   Once message goes away, it appears possible to close pdf, and re-open it (or new ones) without message occurring.  However, restarting (turn off then on computer) causes message to occur again on FIRST pdf opened.  Very annoying, and disconcerting to not know what is NOT working.

Adobe - PLEASE FIX.

New update post:  3/5/2026  1630

Yesterday I updated Reader to 2025.001.21265 on Computer One (Dell M2800, Win 7 Pro) which also has loaded on it SumatraPDF, FoxitPDF and Firefox (please remember these).  

A quick summary of the steps on Computer One is:  1)  put missing api..dll’s in appropriate System folders (as per HelpfulJohn).  2)  Downloaded and installed 2025.001.21223  3)  Use Adobe to open pdf.  Open correctly.  4)  After a while Adobe updated itself to 2025.001.21265.  5) disabled api….dll’s (changed names) in Systems files - Reader still worked.

Adobe behaved correctly:   1)  No “missing api...dll” message   2) No (not one) error message about WinHttpWebSocketCompleteUpgrade missing from dynamic link library WINHTTP.dll   3)  Using “Open With” to open a pdf, all four PDF readers listed above were shown as available, and they were all in the Windows Default Programs list for selecting default program to open pdf files.

Everything seemed to work as it should for Computer One.

---------------------------------

Today I updated Reader to 2025.001.21265 on Computer Two (Dell E6520, Win 7 Pro) - different story:

To do this update I removed Reader Version 22.003.20282, earlier suggested by one participant as working on Windows 7.  It had only kinda-worked for me (several issues).  You may have some other older version  installed - it doesn’t matter.

Steps:  1)  Downloaded 21223 install file from site given by Helpful John.  2)  Downloaded Acrobat Cleaner tool from https://www.adobe.com/devnet-docs/acrobatetk/tools/Labs/cleaner.html.  3)  Used Windows Programs and Features to uninstall older Reader (DO THIS FIRST), then Cleaner tool to remove all traces of old installation.  4)  Put each api….dll into appropriate System folder.    5)  Re-boot computer [precaution only].  6)  Install 21223 from executable . 7)  Download from https://www.adobe.com/devnet-docs/acrobatetk/tools/ReleaseNotesDC/continuous/dccontinuousmar2026.html#dccontinuousmartwentytwentysix   the 32-bit update file AcroRdrDCUpd2500121265.msp.   I believe (but am not sure) that this file will only update 2025.001.21223.  Click to install.  You should now have a “working” Version 21265.

BUT…. for Computer Two:  1)  when I opened a pdf using the 21265 updated file, I still got the error message about WinHttpWebSocketCompleteUpgrade missing from dynamic link library WINHTTP.dll .  Clicked OK three times to make it go away.  It kept coming back at random times  2)  SumatraPDR and Foxit no longer appeared in the “Open With” option list, nor did they appear in the Default Programs list for pdf files.  I had to uninstall/install SumatraPDF to get it back in the list.  On another computer with updated 21265 Adobe, Sumatra and Firefox pdf options (but not Foxit), Sumatra and Firefox went away from the lists. 

IMHO Adobe has not fully fixed the 32-bit Reader (still get the  WinHttp..Upgrade...HINHTTP.dll error message - randomly, on some of my computers but apparently not on others - and the disruption of “Open With” options).  I have no confidence in Adobe Reader despite Adobe’s effort to fix it.  For now I will use SumatraPDF.  It does not all the fancy features of Adobe, but it seems to correctly open, read and print the pdf files I tested it with.  And...it uses a fraction of the computer resources that Adobe uses.  Firefox has a problem printing pdf files which look correct on the screen but do not properly fit the page when printed.  I reported this to Mozilla a couple of days ago, and from the response I got they are looking at it.  We’ll see.

I hope my experience is of some help.  I want to thank HelpfulJohn for the effort he has put into this issue.

How do I fix Acrobat Reader error on Win 7: “api-ms-win-core-winrt-I1-1-0.dll is missing from your computer” in detail?
Everyone keeps saying open Reader and check for updates but it will NOT OPEN because of the missing .dll!

Can anyone please give a detailed answer on what to do?

Do I uninstall Reader or not?

Please post the link to install the update and name the update please because there are four of them in the list.

Thank you if anyone will take the time to post a detailed reply.

Hola AnandSri:  resulta que cuando empezaron todos los problemas, desinstalé el programa para poder comenzar todo desde el principio correctamente con la actualización.

He estado esperando respuesta para que se lanzara la actualización que indicas y por tanto ahora no puedo entrar a instalar la actualización, porque resulta que cuando intento instalar el programa Adobe Acrobat reader, sale un mensaje de error (W64 bits): se interrumpe, “codigo de error 150201” con lo que si no puedo instalar el programa, no puedo descargar la actualización y no puedo utilizar.

¿podrías ayudarme por favor?

Muy agradecido