Thanks for your question. Can you confirm whether you are using Document Manager as well?
The most common cause for PDF's not being able to open from AO would be because of the "enhanced security" in Adobe Acrobat Reader - you can disable this to confirm. In Acrobat Reader, go to Edit > Preferences > Security (Enhanced) and disable the "Enable Protected Mode and Startup" and "Enable Enhanced Security" and see if this resolves the issue.
Matt Practice Solutions Product Specialist MYOB Australia Pty Ltd