If it is being processed by an Exchange server or going to an Outlook user, it is probably helping out as the recipient might not be able to save the pdf (although that bug in Outlook may only apply to jpegs or other graphics).
There are various ways you can try to format the email to overcome the problems with Outlook. Many people suggest Attachment Tamer (~$15). However, you can just send Plain Text and add the attachments to the end of the message (Edit menu, Attachments), you can change the font to something other than your default font, or you can attach a blank text file at the end of the message (which seems to be happening for you anyway).