"You can get viruses from a PDF?" you say??
YES you CAN!!! and Seth Hardy of Symantec's MessageLabs just did a talk the other day at SecTor 2009 about how he's been able to (in a test lab) create a virus, embed it in a PDF and get past every known antivirus. This is scary stuff folks, and there is one little thing you can do to stop most of it.
Here is how you do it in Reader 9.1.x :
Click on the Edit menu, click Preferences.