PDF Preview and Download Error in New Outlook - Adobe Acrobat Reader Says File is Damaged or Unsupported

This appears to be a bug in the "New" Outlook app which boils down to how the app caches email attachments on the local PC.

Attachments are downloaded and cached with an unusually long file path in the following location: C:\Users\USERNAME\AppData\Local\Microsoft\Olk\Attachments, and then within further folders named with some sort of GUID, which can potentially hit the upper path length limit in Windows.

I've observed that certain PDF files (typically those larger in size) are cached to the local disk in this location rather that being previewed in memory, and then they become corrupt likely due to the long file paths nested several folders deep within AppData. Even setting the LongPathsEnabled registry key does not seem to solve the issue. Disabling the "Offline" option in Outlook settings has no effect either.

The only workaround that has worked for me is taking ownership of the C:\Users\USERNAME\AppData\Local\Microsoft\Olk\Attachments folder, removing ALL permissions, and setting the folder to Read-Only so the Outlook app can no longer write anything into this folder. This forces PDF attachments to be loaded and previewed without being cached locally first.

This is definitely not an Adobe issue, but rather an issue with the "New" Outlook app which appears to have been recurring for quite some time now. Hopefully Microsoft can find a true fix to this behavior instead of having to rely on workarounds.

Hi,

What solved the problem for me was to redefine Adobe Reader as Default PDF Handler (Even if it's already set):

• Close Outlook
• Open Adobe Acrobat Reader
• Go to Menu > Preferences > General
• Click button "Select as Default PDF Handler"
• Let it re-register all file associations
• Restart Outlook and test again

It helped in my case, fo another colleague, it didn't. But I'm using the Pro version of Acrobat and he just the Reader.
You could also check if Acrobat and M365 Apps are both installed in the same bit-version (32-/64-bit).

I tried all methods, but still issue remains the same. Its not happening in new outlook, itshappening in browser aswell.

Dear @Sang Tran

Thank you for posting your question in the Microsoft Q&A forum. The fact that PDFs work correctly in classic Outlook and (presumably) Outlook on the Web confirms that the issue lies specifically with the new Outlook for Windows client. This is a known issue that some users have reported, where the new app appears to incorrectly handle the file encoding during the download or preview process, leading to a corrupted file. Here are the recommended methods: 

Method 1: Disable PDF Previewer in Microsoft PowerToys 

1. Click the Windows Start button, type PowerToys, and open the PowerToys app. 
2. In the PowerToys window, select File Explorer Add-ons from the sidebar. 

1. Locate the option labeled Enable PDF (.pdf) preview and turn it off by deselecting the checkbox. 

1. Close PowerToys, then fully close and restart Outlook. If possible, restart your computer to ensure the change takes effect. 

Method 2: Match Application Bitness (32-bit vs. 64-bit) 

Outlook and PDF viewers like Adobe Acrobat Reader must match in “bitness” (32-bit or 64-bit) for previewing to work. If the versions differ, Outlook cannot load the correct preview handler, resulting in the error. 

For a comprehensive guide from Microsoft on this issue, please reference this official document: Outlook PDF preview error: This file cannot be previewed because there is no previewer installed for it - Microsoft Support 

Method 3: Report the Issue to the Outlook Team (Most Important Step) 

Since the new Outlook is still under active development, providing feedback directly to the engineering team is the most crucial step for a permanent fix. Your diagnostic data will help them resolve the bug in a future update. 

Since user feedback plays a critical role in shaping Microsoft's product roadmap, I urge you to share your experience directly with the product team through the Microsoft Feedback Portal. This will ensure that your opinion is heard and help prioritize improvements that matter most to users like you.  

Here's how you can give your valuable feedback: 

• Visit the Ideas Community 

Select "Send Feedback" and describe your experience in detail.

• You can also search for existing suggestions and upvote the ones that match your concern. The more votes a proposal receives, the higher its visibility and priority. 

Thank you for your patience as Microsoft continues to improve the new Outlook experience. 

If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".   

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We had the same exact issue today with a single user and I discovered our firewall gateway-av (Sonicwall) was blocking the files because it thought they were zip/compressed. Thus far, I've not found a solution.