I think it is not FOPE which have tampered the attachment. Please update me the Configuration of the Inbound Mail flow and the Components involved in Mail Delivery for your domain where you received that attachment.
I would also like to know which subscription have you subscribed for FOPE (BPOS, Office 365 or FOPE for Onpremise Mailing Solutions).
I would also like to know was there notification sent you by Microsoft or Forefront regarding this action taken on that PDF file?
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