word of warning - pdf spamming is a new form which is taking over from Image spamming... most anti-spam tools wont be able to detect these for some time I believe...
I think the main rationale is to hide the text from spam filters so that they get through. However I'm not sure if PDFs can contain "active" content that might be able to automatically connect back to the internet etc. to confirm a positive hit or do worse for example... I think that some of the results in the Google search that I posted earlier goes into more detail about the rationale and capabilities of such spam.
There was a PDF Virus before Peachy Virus (2001). However it relys on a exploit with in your PDF reader software. No idea if someone found a new exploit. We'll find out in time.
Put your email through a google account. Its gets rid of more spam than anything else I've seen. You can redirect it to your POP3 account.
Personally if I don't know the email subject or sender, and it gets through it gets deleted immediately.