I am the IT administrator for several or our users, on Windows and macOS, using both Adobe Acrobat Reader and Adobe Acrobat Pro.
Recently, it has been brought to my attention that if users on Windows10 digitally sign a PDF (using a valid and trusted digital certificate) and if they send this new digitally signed PDF via email (Outlook), users on the mac side are not able to open/preview it from Outlook, even if we first "save as" the file and then open with Adobe. The error message is always the same "There was an error opening this document. The file is damaged and could not be repaired."
We have shared the exact same digitally signed PDF via other mediums, and the file then opens just fine with Adobe Reader and/or Pro.
We can also open the file with Outlook Web App, any browser on mac.
I see that back in 2016, Adobe posted a fix for a similar issue, and I have tried these steps but still no go.