Adobe and Yahoo announced a partnership to be the first to bring ads into PDF documents.
An excerpt from AP:
For example, a company that publishes a daily bulletin for electrical engineers may open the content to digital camera companies, laptop computer vendors and other advertisers. The ads will contain links so the reader can go immediately to the advertiser's Web site.
The new system works by letting publishers upload their documents online and Adobe scans the files to insert ads. Ads cannot be placed by choice, but those from competitors can be blocked. Also, ads don't get printed, and the service is available for U.S.-based publishers only.
The move gives Yahoo a one-up on Google and Microsoft for a change and also marks Adobe's entry into the ad space.
Yahoo, Adobe team on PDF ads (News.com)
Adobe And Yahoo Partner To Put Ads In PDFs (InfoirmationWeek)