If the PDF Preview Handler is not working in Outlook 365, it is usually caused by one of the following reasons:

Adobe Acrobat Reader or another PDF reader is outdated or corrupted.
The PDF preview handler is disabled in Outlook.
A recent Microsoft Office or Windows update caused a compatibility issue.
Protected View or Trust Center settings are blocking the preview.
The Outlook profile is damaged.
The PDF file itself is corrupted or password-protected.

Start by restarting Outlook and your computer. Then make sure Adobe Acrobat Reader is updated and set as the default PDF application. You can also check Outlook > File > Options > Trust Center > Trust Center Settings > Attachment Handling to ensure attachment preview is enabled.

If the issue still persists and your Outlook data file (PST) appears to be corrupted, repairing the PST file may help. If the file cannot be repaired or you need to recover mailbox data, you can use the Advik PST Converter to open the PST file and export emails and attachments to formats like PDF, EML, MBOX, or other file types without relying on Outlook's preview feature. This can be useful for accessing important attachments while you troubleshoot the preview handler issue.

Learn More: https://www.adviksoft.com/blog/fix-pdf-preview-handler-in-outlook/