The ad engine we use on our sites can only use images, text or flash movies for their display. None can use PDFs as they do not render within the site, and the ad engine does not accept them. The ads can be linked to a PDF like we will be doing in your site by uploading the PDF to the server and linking the ad to the uploaded PDF document. There are a few newspaper sites we have built that have another category for their e-editions where they upload their classified pages and similar. That would work too.