"CoolType invalid font error" in Reader in only some files

Question

My Adobe Reader defaults about fonts seem damaged. Any help appreciated.

I have both Reader and Acrobat Pro 2020 and have been exploring the topic of embedding fonts. Something I did has caused the following symptoms.

Problem
Behaviour of Reader with any PDF file that I either download from somewhere new, or is an old (not fixed) file on my computer:

Open any above PDF file in Reader. [Very latest version, I even uninstalled and installed new.]
Click on the Comment Icon.
Click on the bare letter 'T' in the Comment tools line to add a text comment.
Click somwhere in the document.

Error popup prevents Comment Text Add, saying:

"CoolType invalid font error 'cannot fetch default design vector' for font unknown."

Exit Reader without saving.

"Fix" problem for this one file only

Now open any above PDF file which gives the error in Reader in Acrobat Pro 2020.
Click on the Comment Icon.
Click on the bare letter 'T' in the Comment tools line to add a text comment.
Click somewhere in the document.

No error occurs.
Able to add a few characters as a text comment.
Can save the modified document.
Now exit Acrobat Pro 2020.

The document is now permanently fixed as shown by:

Open the modified file in Reader

Add Comment Text now works with no error.
Can now delete the Comment text added by Acrobat Pro 2020.
Can save the document, which remains fixed.

I can even have two PDF files open in Reader at the same time, where one gets the Add Text error and the other now does not. Thus it seems? to be some default assigned to newly opened files?? Is this some default within Reader itself I can change?

QUESTION
How can I prevent Reader from having this error on any new PDF file? Yes, I can fix it because I have Acrobat Pro 2020, but how annoying!!

By the way I saw the post about deleting files with names starting with ~FontCache in
%WinDir%\ServiceProfiles\LocalService\AppData\Local
but there are no such files in that folder so that posted fix for this error does not apply.

HELP???

Any suggestions would be GREATLY appreciated.

mjhanna · Accepted Answer

SOLVED!

After considerable testing it appears that Reader stores "somewhere" the default properties of an Add Text Comment. [Does anyone know where?] This default seems to be stored for "this user" since it remains even after uninstalling and reinstalling Reader. I am "guessing" that this default is only used if the PDF itself does not have any Add Text Comment, since my adding and saving a Comment in a PDF via Pro 2020 allowed that file to now open in Reader with the properties of that added Comment.

How I fixed my problem

I opened a PDF that would allow adding a Comment since it already had a Comment (with valid properties) added via Pro 2020. To reset the default properties for Reader for my user I needed to do three actions. ALL are required:

CHANGE the properties which were presented from having a Pro 2020 Comment. Change anything: font, size, color... Just be sure the font chosen is a valid static font definition on this system.
ADD some/any Text with these new Comment properties.
SAVE the file with these Comments.

The default properties for Comment Add Text has now been changed in Adobe Reader to the above properties for this user. Now Reader will open any PDF which has no Comments without the Add Text action causing the Subject error.

My guess as to how this error situation was introduced

I was testing various custom fonts in Pro 2020 for use as Comment text and viewing my tests in Reader. I am "guessing" that I did the above in Reader at some point which changed its default to an Add Text test font family. I am "guessing" that I decided against that font and uninstalled that font from my system. That left Reader's default properties using a non-existent (and thus invalid) font, which caused the error. But the Pro 2020 default had been changed to a (still) valid font. Thus I presume adding a Comment with valid properties allowed that PDF to use Comment Add Text properties which were valid, and thus open the properties box with valid properties to allow changes. I am also "guessing" that if a Comment exists in a PDF with a font not installed on this system, and that font definition is not embedded in the PDF file, that Reader will convert that font to some fallback font. But I have not bothered to test this (yet).

This is my best "guess", so hope my experience and discovered solution might be of use to others.

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