Does this issue happen to all the users or only one specific user?
Please disable the third-party anti-virus and Firewall on the Outlook client and run “Outlook /safe” to start Outlook with no add-ins and
check whether the issue persists.
So you can send other files of same and bigger size but just not the PDF.
What about there users in the org can they send the PDF.
Try to send using the OWA.
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