Possibly it's some kind of font problem.
I've done a quick search on the net, and someone suggested to turn off "Use device fonts". Print it as an image, download a new version of the PDF reader, or install another PDF reading program; that's what comes to my mind. Didn't find much about it.
I found this too:
http://forums.adobe.com/thread/391763

Everytime I see, hear or experiment weird problems I like analog much more.