Well, if I *had* to guess I would say watermarked .pdf, if the went the .pdf route. So as they are able to track back to the original person and ensure they are punished for copyright infringement. I'd also venture to say that they might choose the Kindle Fire as an electronic device to view their books because they could be done in a proprietary format as opposed to the .epub which normally does not use restrictions to view content one has purchase but is able to do so if they chose to.