This is not a valid Portable Document File ....

first of all, go Preferences > Performance and if you have the next alert message:"An integer between 96 and 8 is requiered. Closest value inserted."
So this would be the solution for both problems "This is not a valid portable document file (PDF). It cannot be opened" and "An integer between 96 and 8 is requiered. Closest value inserted."

In Regedit:

\HKEY_CURRENT_USER\Software\Adobe\Photoshop\120.0

Added DWORD "OverridePhysicalMemoryMB" with the value "24000" decimal bas (=24 GB RAM = my installed RAM).

I was able to open the Performance tab in preferences now. And also tools like "blur" are working.

can't copy from can't copy illustrator to photoshop !

" this is not a valid portable document file pdf document. it cannot be opened "

Here it is, still showing up, I guess -- I'm on PS CC and have encountered this same problem. Can't create a new document without having this message show up. Tried the Restore Default Workspace and other recommendations. Still happening.

I just encountered this issue today. It happens every time I launch Photoshop and try to open a new document.

Using:

Photoshop 18.0.1

macOS 10.12.3

Had this error twice and followed all of the advice on this forum.  Completely disinstalled and then clean reinstalled Photoshop elements 11, and still got the same error.  Weirdly, removed the shortcut icon from the Dock on my Mac and didn't get the error...until I was editing a screenshot and got the error again.  This is a complete pain, and makes the software unusable.

I GOT PROBLEMS TOO...got this error when trying open the pdf files in android devices "The document cannot be opened because it is not a valid PDF document"  ..im using samsung s4..i tried open the same files at windows pc and its can be open fine,the files not contain any passwod btw..seem some files cannot be open at android is this normal or what? got solution of tht?or i just search another pdf viewer?

example of files http://www.mediafire.com/download/raz3x0bk1mvnni4/Lawak+Kampus+01%40ihazaliza.blogspot.com.pdf

this the screenshot

I'm having this same problem.  I did two things that might be related:  I installed a trial version of InDesign, and I clicked on an auto-update for Adobe Reader.  Now, anytime I launch PSE 8 -- either directly from the dock, or through Bridge -- I get an alert box informing me that "This is not a valid Portable Document File (PDF) document.  It cannot be opened."   The files I'm opening are DNGs.  After I clear the alert box the first time, the files open -- but the alert comes back every time I perform any action at all.  I'm on a Mac, running OS 10.5.8.  I've tried clearing the preferences, and it didn't work.  Any help is appreciated, since I use PSE on a daily basis and this is making it, for all practical purposes, unusable.

No. Try deleting preferences. Quit PSE, then restart while holding down command+shift+option. Keep holding the keys till you see a window asking if you want to delete the settings file. You do.