I got a Spam email yesterday that just comprised of a pdf attachment not text or html content, Mailwasher picked it up as spam, but I wanted to check it out as I too felt this could be a new vector.
Saved the attachment, scanned by avast and VT no detections the contents of the pdf was basically an image (with a general spam text message saved as an image) and no embedded links. As a matter of fact it was such a poor spam there was no url in the image text a real plonker.