After reading about it on several websites, I finally got my first PDF Spam message. The message itself was empty, with only a simple PDF document attached. The contents of the PDF is a single image, the same type you often see in image spam:

It has misaligned fonts and different colors to fool OCR software, and the PDF itself was damaged. PDF readers had no problem opening the document but some other tools complained about broken references and refused to work. Obviously those tools were not written with spammers in mind.

The message came through my ISP's spam filters undetected.

I checked our filters at work and there a few of these messages were blocked by the antispam engine. It didn't seem to have a problem detecting them, but there were only a few, maybe it just got lucky ;)