There's an old saying that advertising abhors a vacuum. That's why you see billboards and ads covering the sides of roads, buildings, and even T-shirts. Anywhere there's an inch of unused space, you can be certain someone is plotting a way to insert an advertisements.

So perhaps we shouldn't be surprised that Adobe and Yahoo! are partnering on a method to insert targeted advertisements into PDF documents. Not just the ones you read online, but even PDFs that you download to your computer.

The announcement doesn't mean that every PDF file you read from now through eternity will have an ad tacked onto the side. But it does mean that content publishers now have yet another way to try to convince you to buy stuff. If you download or open an infected file with advertisements online, you'll see a clickable advertisement that will open up a new page in your default web browser.

Publishers can sign up for the Adobe PDF Powered by Yahoo! program and then upload their PDF content to be ad-enabled before it's distributed. According to Adobe's press release, ads will only show up in Adobe Reader and Adobe Acrobat, so it's possible that you may be able to avoid advertisements by using a third party application like FoxIt.