Anyone with Adobe Reader installed on their machines (which is probably a majority of PC users) should be aware that the company has patched a recently reported vulnerability discovered in the software. At least for the latest version of the reader, Adobe has issued a patch that fixes the "JBIG2" security hole. Originally reported towards the end of February, it's been a widely covered and fairly serious hole that affects versions of Adobe Reader from 7 onward. Those using older 7 and 8 versions are still vulnerable, but Adobe does plan to introduce a patch for them within the next few days.
Given that many people consider PDFs to be a safe medium, it doesn't take too much speculation to see how this flaw could have been abused. It is a cross-software flaw, affecting both Adobe's software and some third-party applications as well. If you're using Adobe Reader 9, make sure to get the patch.