Adobe are working on how to have paid ads in PDF documents, similar to Google AdSense ads on web pages. The new system is called "Ads for Adobe PDF" and is in beta. There is a video,
View the demo above and check out the sample screenshot.
FAQs and an application form . The system tries to allow for PDF's lower interactivity than web pages (the ads are fixed in the PDF document, but only for a limited period). The service is being provided with Yahoo! who already have a web based advertising service. There is also an Ads for Adobe PDF discussion forum. It will be interesting to see how well this service is accepted. It could be a good way for publishers to be recompensed for the cost of providing material. The current service seems more targeted at publishers might also be useful for self publishing authors.