PDF has contextualized advertising.

Adobe and Yahoo have developed a technology that will allow the implementation of contextual pay-per-click advertising in PDF documents, it is now in beta testing. Publishers and the media will greatly benefit from this invention. Now they can upload free e-books, electronic versions of magazines and newspapers in PDF format, making a profit from advertising.

In addition, the new technology should be of interest to commercial companies that publish press releases, user manuals and other materials on the Internet in large numbers. With the help of banners, they will not only earn a penny, but also be able to track the fate of files after they are distributed on the Web. The PR department will receive accurate statistics on the popularity of each document (probably, even the number of views, and not just clicks on banners).

Contextual advertising is implemented by a vertical column in the right part of the document, as seen in the illustration. To insert an advertisement, you need to upload the document to the Yahoo server, and after a few minutes, take it back with the advertisement. Deductions from the transitions on banners get Yahoo and Adobe.
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via CNet