Did you test with 3.7.21, or are you assuming that because of the post Has PDF handling changed in 3.7.21? Please test if you haven’t. That is, get f.ex. 3.7.20 and verify that it works for you there, then progressively install newer releases until you find when it stops working (Note, this will not affect any of your downloaded modules or saves, as long as you do not tinker with those).
Well, in the cited post, the PDFs were shown - just not in a PDF viewer but in a Web-browser. What you seem to say, is that the PDFs are not shown at all.
Did you run the tests described in this reply? If so, can you report the results?
Please do. It might contain useful information.
Then why report it the problem here in this Forum, and not some Windows help-line?
Embedded PDFs are extracted from the module into some some temporary directory. Then Vassal uses java.awt.Desktop#browse to direct the Desktop to open up the file. How that message is handled both by java.awt.Desktop#browse and Desktop depends a lot on the Operating System and the Desktop (Windows+Windows, MacOS+Cocao, Linux+Gnome/KDE/…). In older OpenJDK, java.awt.Desktop#browse on Windows would execute start <filename>, but now it tries to find the default (defined in the Desktop) application to on a URI of the format file:/.../....pdf. For some reason, what it seems to find in some cases is a Web-browser, but perhaps in your case - nothing.
If you run the mentioned tests cited above, you may learn more.
Yours,
Christian