Mail Security unable to scan emails with PDF attachments

I am running Mail Security for Exchange 7.0.0.63 on Exchange Server 2007. A large number of emails with PDF attachments routinely get quarantine because mail security is unable to scan them, yet other emails with PDF attchments are ok. Because of this problem I've been forced to set the Unscannable Rule to "log only", which obviously increases the risk to my users.

Is this a "bug" in the software? If so, is there a fix? 

Thank you