According to an article I got from my boss, there is a flaw in Adobe Acrobat and Adobe Acrobat Readers that will allow hackers to take over your machine if you open a PDF file they send you. The article only mentions Windows XP Service Pack 3 as a vulnerable OS, so it’s unclear whether Vista can be taken over this way as well. Despite how serious this flaw is, a fix for it won’t be available until March 11. Between now and then, you can either swear off the opening of PDFs entirely, or use an alternative PDF reader.
Those of us using Mac OS X (which understands the PDF format natively) need only make sure that Preview or Safari is the default PDF reader.