There is an interesting development in Yahoo and Adobe collaboration, when they recently launch new ads program called Ads for Adobe PDF powered by Yahoo! beta program. This ads however only available for U.S. publishers of English-language content, initially.

Watching the demo provided on Adobe lab website, the PDF is not automatically filled in with ads, but it need to be uploaded to their system prior that

When the files uploaded to the net after that, it works like contextual ads, where the whole content will be scanned before relevant ads being displayed.

This should be an additional opportunity to earn more income especially those who have a lot of PDF files displayed all over the internet. Browse over to their website for the registration