There are dangerous PDF files being circulated by spammers.  The new PDF based attacks typically use Javascript attacks within the document to infect vulnerable systems.  Users should always avoid opening any unexpected document or link in email messages.  Also, it is important to stay up-to-date on all security updates available from Adobe and other software vendors. 

Malicious PDF files - Death of the Internet in 2012
http://blog.trendmicro.com/death-of-the-internet-foretold/

The malware involved in this spam run is detected by Trend Micro as TROJ_PIDIEF.JT, a Trojan that arrives as a PDF file named DOC.PDF. This file promises more information regarding the alleged Internet death.

PIDIEF Trojans are known malware droppers or downloaders, so once users click on the attached PDF file — and whether or not they believe the theory — another malware is already up and running on their systems and doing malicious routines. The death of the Internet is going to be the least of their problems after that …

Internet Storm Center - PDF Javascript based exploits
http://isc.sans.org/diary.html?storyid=4726