New Outlook - Can't Open PDFs

Test the same shared-mailbox message in Outlook on the web, then use Save As and open the downloaded PDF; if either works, use that path temporarily and escalate the failure as a New Outlook client issue rather than repeating the %LocalAppData%\Microsoft\OLK folder reset.

Isolate the failing component

1. Open the shared mailbox in Outlook on the web and test the exact same PDF.
   • Works in the browser: Exchange and the attachment are accessible; continue with the New Outlook checks below.
   • Fails there too: test another user with shared-mailbox access. If multiple users fail on the same attachment, investigate the mailbox/service side.
2. In New Outlook, select the attachment’s drop-down menu → Save As → save it to a local folder.
   • If the saved file opens normally, the PDF itself and its Windows viewer are working; the failure is in New Outlook’s preview/open handoff.
   • If it does not open after downloading, test another known-good PDF and check the Windows PDF association.

New Outlook officially supports saving attachments through Save As and uses the default browser or application to open or preview many attachment types.^1^

Check the PDF association

Microsoft documents Microsoft Edge as the expected handler when it is the default browser for New Outlook PDF attachments.^1^

1. Open Windows Settings → Apps → Default apps.
2. Search for .pdf.
3. Temporarily assign Microsoft Edge.
4. Close and reopen New Outlook, then retest the shared-mailbox PDF.

Windows supports assigning a default application by file extension from that location.^2^

If Edge works but Adobe does not, repair or reinstall Adobe and then restore Adobe as the .pdf default. If neither works from New Outlook but both open the downloaded file, retain that result for escalation.

Check tenant status and capture escalation evidence

There is no matching entry on Microsoft’s public New Outlook known-issues page for an intermittent failure opening PDF attachments specifically from shared mailboxes.^3^ Similar shared-mailbox attachment failures have nevertheless been reported.^4^

An administrator should:

1. Open Microsoft 365 admin center → Health → Service health.
2. Review active and recent advisories for Exchange Online and Microsoft 365 apps.
3. If no advisory applies, open a support request and include:
   • New Outlook version, client version, and WebView2 version.
   • Whether the exact attachment works in Outlook on the web.
   • Whether Save As succeeds and the downloaded PDF opens.
   • Whether the issue follows the user to another device.
   • Whether another delegate can open that same shared-mailbox attachment.
   • Timestamp, sender, shared mailbox, and message subject—without attaching sensitive content unless support requests it.

Service Support Administrator and Helpdesk Administrator roles can view service health, including active incidents and recent issue history.^5^

Avoid these unrelated fixes

• Do not repeat the OLK\Attachments rename; that cache reset has already been completed.
• Do not apply classic Outlook PDF preview-handler, Trust Center, Office/Adobe bitness, or Level1Remove registry fixes unless the problem is reproduced in classic Outlook with the documented previewer or blocked-attachment error. Those procedures target different Outlook clients and symptoms.
• Do not change attachment-security registry values merely because the file is a PDF. That lowers security and is intended for explicitly blocked file types or linked attachments, not this intermittent shared-mailbox behavior.

Until Microsoft support identifies the client-side cause, Outlook on the web, Save As, or classic Outlook—if available and confirmed working—are the lowest-risk workarounds.

References

1. Troubleshoot attachment problems in Outlook for Windows
2. Change Default Apps in Windows | Microsoft Support
3. Fixes or workarounds for recent issues in new Outlook for Windows ...
4. Attachments in Shared Mailboxes won't open, preview or save.
5. How to check Microsoft 365 service health