Cant open PDF files

Hi

We receive PDF document via email. We can open PDF - all good. We upload PDF to SharePoint, we can open the PDF in SharePoint - all good. The file can be opened several times in SharePoint until suddenly an error message appears saying  couldnt open - not supported file type, or file damaged. 

Have spent nealy 3 hours in 2 Microsoft calls to try to resolve - they checked everything and couldn't fix. MS said call Adobe. Can't log call for actual support from Adobe without a plan, and dont understand on line solutions offered by Chatbot.  This is only happening to 2 out of 10 users who are all on same Office 365 Business Standard plan. Lap tops have been checked, software, software updates - all checked by support technician and via MS support - so am asking for help and guidance from community. Any help, guidance welcome. Thanks!