Yes it is TRUE.
Now you can earn money by displaying contextual ads on your pdf documents. It has been made possible by a combined effort of Adobe with Yahoo. You have to use a special plugin "Ads for pdf" of Adobe PDF Acrobat 9, which will allow you to serve context based ads related to your content, when any user reads your pdf document in his PC. If he clicks on these ads, then a browser window opens and he is taken to the advertisers website. The advertisers pays money, and you are given a share of that.
Ads for Adobe PDF is a new opt-in service that enables publishers to monetize their PDF content through advertising. The advertising service, powered by Yahoo!, automatically matches and displays dynamic, contextual ads in Adobe Reader and Adobe Acrobat when a user views the PDF. Publishers that sign up to participate in this program are eligible to be paid for valid clicks on ads. In this unique advertising relationship, Yahoo! helps connect advertisers from its extensive network of online advertisers with publishers that are distributing content through Adobe PDFs. Yahoo! will incorporate this new advertising inventory into its portfolio of online advertising opportunities.
This will surely pave a new income stream for offline publishers and content developers.
Visit Ads for pdf