Can't preview files in Outlook

Close Outlook and check if you have Microsoft PowerToys installed. If you do, its PDF previewer can cause conflicts. Follow these steps:

Microsoft PowerToys > File Management > File Explorer add-ons > Preview Pane > turn off Portable Document Format (PDF)

Other fixes:

In Outlook:

• Go to File > Options > Trust Center > Trust Center Settings.
• Click Attachment Handling.
• Make sure “Turn off Attachment Preview” is unchecked.
• Click the Attachment and Document Previewers button.
• Ensure that “PDF Preview Handler” is checked.