When users open a message in its own window that has a pdf attachment and replies or forwards the message; when they close the original window, outlook prompts "The properties of the message <email_subject> have been changed. Want to save your changes to this message?"
Steps to recreate this issue:
Message must have pdf or txt attachment
Open the original message in its own window
Reply-to or forward the message
Closing original message that was previously opened in its own window
Attempts to disable add-ins or running outlook in safe-mode do not resolve this issue.