Microsoft outlook is corrupting PDF and office files- replacing attachemnts with a 4k file which cannot be opened.

The behavior described (attachments suddenly becoming ~4 KB and no longer opening, including when viewed in Outlook on the web) matches issues caused by third‑party antivirus or security software interfering with attachment handling and preview, rather than Outlook permanently rewriting the file contents.

From similar reported cases in the context:

• Turning off real‑time scanning in McAfee resolved issues where Outlook classic altered and broke PDF files when previewing them.
• Another user with attachment download/open issues confirmed the problem started after an antivirus update and was resolved once that was fixed.

Given that the attachments now appear as 4 KB in the mailbox (including in Outlook on the web), there is no supported Microsoft mechanism to restore the original attachment content from Outlook or Exchange once the server copy is already in that state. Recovery options are therefore limited to:

1. Check for other copies of the messages
   • Look for the same emails in:
     • Other mailboxes or folders (e.g., archives, PST/OST backups, or other accounts where they might have been forwarded before the issue started).
     • Any local or server backups maintained by the email provider or organization.
   • If the account is managed by an organization, contact the IT or Exchange administrator to see if they can restore the mailbox or specific items from backup to a point before the attachments were altered.
2. Stop further corruption and prevent recurrence
   • Temporarily disable or adjust real‑time scanning in the antivirus/security product that is installed on devices used to access Outlook, especially if it is McAfee or another product known to interfere with Outlook attachments.
   • Work with the antivirus vendor’s support to obtain an update or configuration that no longer interferes with Outlook attachments. In the examples provided, users escalated to McAfee support and left real‑time scanning off while the vendor investigated.
   • Until the antivirus issue is resolved, avoid forwarding or opening critical attachments from Outlook desktop or mobile on affected devices; instead, use Outlook on the web from a clean environment that does not have the problematic antivirus installed.
3. Ask senders to resend the original files
   • Where possible, contact the original senders and request that they resend the attachments.
   • To reduce the chance of blocking or corruption, senders can:
     • Upload files to OneDrive or another cloud storage and send a link instead of attaching the file directly.
     • Compress the files into a .zip archive before attaching them, which is a recommended workaround when dealing with blocked or problematic attachments.

If the attachments are already reduced to 4 KB on the server and there are no backups or alternate copies, Outlook and Exchange cannot reconstruct the original files.

References:

• Blocked attachments in Outlook
• Open, save, and edit attachments received in Outlook
• Outlook blocked access to the following potentially unsafe attachments
• "You don't have appropriate permission to perform this operation." message when I try and open an attachment in Outlook - Microsoft Q&A
• Attachments not downloading - Microsoft Q&A
• M365 Outlook Classic alters PDF files - Microsoft Q&A