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RESOURCES · MICROSOFT 365 & OUTLOOK

PDF files won't preview
in Outlook or File Explorer?

You click a PDF attachment in Outlook and the reading pane says "This file cannot be previewed", or the File Explorer preview pane stays blank. The PDF opens fine when you double-click it — it's just the preview that's broken. Here's how to switch the previews back on.

Applies to: Outlook · Windows · Adobe Time: ~10 minutes Level: Confident user / IT
Quick answer

In Adobe, go to Edit → Preferences → General and tick "Enable PDF thumbnail previews in Windows Explorer", then set Adobe as your default PDF app. Turn on the Explorer Preview pane, and make sure Outlook's attachment previewers aren't disabled in the Trust Center.

WHY IT HAPPENS

The preview handler is switched off, not the PDF.

Both the Outlook reading pane and the File Explorer preview pane rely on a "preview handler" that your default PDF app registers with Windows — usually Adobe. If Adobe's preview options are turned off, Adobe isn't set as the default PDF app, or Outlook's Trust Center has attachment previewers disabled, the preview fails even though the PDF itself is perfectly fine. It's almost always a settings problem, not a corrupt file.

THE FIX

Work through these in order.

  1. Set your PDF app as the defaultOpen Windows Settings → Apps → Default apps, search for .pdf, and set it to Adobe (or Microsoft Edge). The preview handler comes from whichever app owns the .pdf type.
  2. Enable Adobe's preview optionsIn Adobe, go to Edit → Preferences → General and enable the PDF thumbnail / preview options (including "Enable PDF thumbnail previews in Windows Explorer"). Click OK.
  3. Turn on the File Explorer Preview paneIn File Explorer, go to View → Show → Preview pane (or press Alt+P), then click a PDF. The preview should appear on the right-hand side.
  4. Check Outlook's Trust CenterIn Outlook, go to File → Options → Trust Center → Trust Center Settings → Attachment Handling. Make sure "Turn off Attachment Preview" is unticked and the individual previewers are enabled.
  5. Repair AdobeIn Adobe, go to Help → Repair installation. This re-registers Adobe's preview handler if it got knocked out by an update or another PDF app.
  6. Restart Explorer and OutlookClose and reopen Outlook and File Explorer — or simply restart the PC — so the preview handler change takes effect. Previews often don't appear until after a restart.
  7. (IT) Re-register the PDF preview handlerIf previews still won't show, IT can re-register the PDF preview handler in the registry, or reinstall/switch the default PDF app. A leftover entry from an uninstalled PDF app is a common culprit.
A 32-bit vs 64-bit Office/Adobe mismatch used to break PDF previews entirely — keeping both current usually resolves it. If previews are missing across the whole team, it's almost certainly a configuration or default-app issue, and IT can push the fix out to every machine at once rather than touching each PC by hand.
THE SMALL STUFF, SORTED

Broken previews are a symptom, not the disease.

When PDFs won't preview, defaults keep resetting and little Office niggles pile up, it's usually a sign your Microsoft 365 and device setup needs proper management. Our free IT Health Check reviews your whole environment — Office configuration, defaults, updates and security — and shows you exactly what to fix so these papercuts stop eating your team's day.

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