I had an issue with PDF files not opening from email attachments. We ended up having to disable Protected Mode in Acrobat Reader. This can be achieved using the following steps.

1. Open Acrobat Reader, got Edit | Preferences (or press Ctrl+K).
2. Navigate to Security (Enhanced) in the Categories list.
3. Unselect "Enable Protected Mode at startup".
4. Close both Acrobat Reader and Outlook.
5. Open Outlook and try opening the PDF file. Should open without any issues.

There is also a Group Policy ADMX file you can download from Adobe to manage this using a GPO. Sorry, I don't have the link of hand.

Regards,

Hugh