Adobe Acrobat DC could not open "--.".pdf because it is either not a supported file type

This error occurs sometime when I attach PDF to my email messages. (My recipients sometimes get this error message.) This only happened when I started using Microsoft Outlook 365. When I use regular Microsoft Office this doesn't happen. I asked a client today to forward me my earlier message with the "damaged" file and I also got the error message when I tried to open it. As well, I can resend the file later on, to the same person using the 365 email address, and it works. (Nothing changed on my end.) It's hit and miss, it seems. I see from posts that this has been happening since at least 2014. Has anyone found a solution? (Besides editing headers, etc. etc. I don't time to do that to every one of my files, especially since some work and some don't.) I use Adobe Acrobat DC (the latest), and Windows 7.

Thanks in advance.

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