Was receiving the same complaint from an end user. Here is what we are trying:
Look at the file name before you save it. My end user has a habit of adding a date to the file name and since we are in the year 2021 she was ending each file with .21
I think this was then converting the file to the .21 extension which was causing the PDF to become corrupted. Hopefully having her change the file name before she saves will solve this issue.