PDFs are being used to bypass Anti-Spam and Anti-Virus detections.  I'm seeing stock related PDF spam daily and users should avoid opening all untrusted email, attachments, or URLs.

PDF Spam Outbreak
http://www.avertlabs.com/research/blog/index.php/2007/06/26/pdf-spam-outbreak/

QUOTE: A large “pump-and-dump” stock spam campaign is underway, but rather than including the content of the spam in an image file, this campaign includes the spam content within a .PDF file. The stock spam is believed to be sent from Stration infected computers, as this spam campaign closely followed a new W32/Stration worm mass-mailing which contained a number of .PDF files, and Stration has been associated with pump and dump spam in the past.

 ISC - Pump and dump scams now in PDF
http://isc.sans.org/diary.html?storyid=3012

QUOTE: Apparently the groups behind what we know as pump and dump spam have found a new way to bypass spam filters. As of yesterday, we’ve been observing e-mails with bogus text, often in german, each with a PDF in attachment.  These PDFs purport to be stock information, and are usually titled ‘German Stock Insider’. They contain much more detail on stock than we’re used to from previous dump and pump scams and include images for added realism.

Kaspersky - Warezov.iq downloader includes PDFs
http://www.viruslist.com/en/viruses/encyclopedia?virusid=147295
http://www.viruslist.com/en/weblog?calendar=2007-06

QUOTE: Earlier today we intercepted a number of mailings with a new Warezov downloader. The good news is that it's already detected as Email-Worm.Win32.Warezov.pk, which we added to our database two days ago. What's interesting about the mails is that along with the usual executable (which in this case is called "access.exe") the messages have a couple of PDFs attached.

Unfortunately, Spam based email continues to increase worldwide:

http://www.postini.com/stats
http://spamcop.net/spamgraph.shtml?spamstats
http://www.messagelabs.com/intelligence.asp