We secure our company data sheet PDFs. Today one of our marketeers told me his email sending said data sheets was returned with this message: "This message contained attachments that were password protected and AKO was unable to scan them for viruses and therefore was unable to accept the message."
So we use Acrobat 9 Pro to password protect. The settings selected are:
Encrypt all doc contents
Restrict editing: printing allowed: Hi res; changes allowed: None.
Do those setting dis-allow virus scanning? If so, can we modify slightly to allow virus scanning?
You could try securing with an older version of the hash, but I suspect whomever is rejecting your attachments will carry on doing so whatever you send them.