Assuming your are viewing it fine in Acrobat or Reader before emailing it, it might be their PDF viewer (like Preview on a Mac, Firefox's PDF viewer, etc.) or it might be the attachment to the email is corrupt. If the former problem, tell them to use Adobe Reader or Acrobat to view the PDFs. If the latter problem, try compressing it (like as a ZIP file) before attaching it to the email. Or upload it to dropbox.com or your own domain and send them a link.
Mike
