Hi Sally GaUS,
As per the description, it seems like the PDF stored in OneDrive will not be opened by local Adobe app correctly as it will prompt the error 'There was an error opening this document. Access is denied'.
If my understanding is correct, to make sure I can have a more overall understanding of the scenario and figure out how we can work together to investigate further and recover the situation, could you please provide more information as required below for a further troubleshooting?
- To narrow down the issue range, please right-click on the PDF files to select Open With option to try if other opening ways like Microsoft Edge will open the PDF successfully.
- If yes, I have searched online and for some other users experiencing the same error about Adobe Reader shows access denied when opening PDF, usually turn off the Protected Mode feature in Adobe Reader will prevent such error from happening. So please give it a try: Protected View feature for PDFs (Windows), Adobe Reader
- If the error persists after you turn off the Protected Mode in Adobe Reader, may I know if you can recall when did the issue start to appear? I believe it used to fine but suddenly it happens recently right? If so, if there have any recent Adobe app update or OneDrive setting change maybe?

Please correct me if I made any misunderstanding above and I will be here waiting for your updates.
Appreciate your time and patience. Hope you a nice day and stay safe!
Best Regards,
Mia