Can't open PDF files attached in Microsoft OneNote (Windows 11)

Hi @Richard Brockbank (Student), 

Thank you for providing the detailed symptoms and version information.

I understand you're encountering an "Access Denied" error in Adobe Acrobat and an "ERR_FILE_NOT_FOUND" in Chrome when attempting to open PDF attachments directly from OneNote. Since other file types like .docx are working and the files open fine when manually saved, this suggests a conflict between OneNote's temporary caching mechanism and your PDF readers' security sandboxing. 

I have outlined the troubleshooting steps to resolve this, including a few quick tests to further isolate the behavior. 

Step 1: disable adobe protected mode 

Adobe’s sandbox often blocks access to the specific temporary directory OneNote uses to "unpack" files. 

1. Open Adobe Acrobat Reader. 
2. Go to Menu > Preferences (Ctrl + K). 
3. Select Security (Enhanced) on the left. 
4. Uncheck Enable Protected Mode at startup. 

Restart Adobe and try opening the OneNote attachment again. 

Step 2: test via OneNote for the web 

To determine if this is a local application or Windows environment issue, try opening the notebook at https://onenote.cloud.microsoft/. 

• If the PDF opens correctly in the browser, the issue is strictly related to the local OneNote desktop cache or the way Windows 11 is handing off the file path to Adobe. 
• If it fails there too, the file may have been synced with incorrect permissions from the source. 

Step 3: temporarily save and open locally 

While you mentioned you have verified the files aren't corrupt, this step serves as a temporary workaround and a diagnostic tool for path-length issues. 

1. Right-click the attachment in OneNote and select Save As. 
2. Save it to a simple location, like C:\Test. 
3. If it opens from there but not from OneNote, the problem is likely that the OneNote cache path is too long (exceeding the 260-character Windows limit) or the folder permissions are restricted. 

Step 4: add OneNote's temp folder to privileged locations 

If you prefer to keep Adobe's security features enabled, you can manually trust the OneNote cache folder. 

1. Copy this path: %LocalAppData%\Microsoft\OneNote\16.0\Cache 
2. In Adobe, go to Preferences > Security (Enhanced). 
3. Under Privileged Locations, click Add Folder Path or Add Host and paste the directory above. 

Please let me know if these steps don't clear the error, and should you have any further questions or concerns, please feel free to reach out.

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