It could be that the PDF file is actually a PDF file within a ZIP file. Try to open the file with a ZIP file utility and see if this is the case. Otherwise two other possibilities:
(1) Whoever sent the PDF file as an attachment used some oddball e-mail client that used some non-standard encoding of the file or treated the PDF file as an ASCII file (PDF files are binary files, not ASCII files) and corrupted the file. If this is the case, have the sender retransmit the file using a service such as Acrobat's Send and Track tool, or Dropbox, or some other similar file transfer service.
(2) The PDF file is actually some type of trojan or virus and whatever was done to make it that way actually corrupted the file. Be very cautious here!
- Dov