Some eReader devices will let readers read PDFs on them, but really the most universal format for now is ePub. (lulu.com)
In order for the free ebook digital copy to be included in the paperback or hardcover version, a single, universal format needs to be agreed upon and compatible with ANY e-reader. (goodereader.com)
The lack of a single, universal format for all devices is problematic, too. (bookwyrmshoard.com)