This is a problem with your email application (if this happens with every PDF file you receive, it's very likely not related to the email setup of the sender). What email application are you using? What operating system?
You could try to just rename the file so that it has a .pdf extension. How you do that depends on your operating system.
Karl Heinz Kremer
PDF Acrobatics Without a Net
PDF Software Development, Training and More...
http://www.khkonsulting.com