After upgrade to Monterey pdf files will not open from Finder

I finally upgraded from Catalina to Monterey yesterday, now *all* PDF files will fail to open with the error, "Adobe Acrobat could not open 'xxxx.pdf' because it is either not a supported file type of because the file has been damaged..." This is for every PDF file. I can open all of them just fine directly from Acrobat. I can also open then as attachments with Preview.

So what broke?