[apage43]

If you use -any- sort of DRM, it will be extremely restrictive to the end user. Personally, I think really what is needed is for it to be convenient for the end user. This is what keeps me buying music from Amazon MP3. I don't even need any software other than a browser if I'm buying a single song. There's nothing to stop me from sharing my files with a few friends but Amazon MP3 probably makes up for whatever sales that would be lost this way because I like the service so much I recommend it to people all the time. Just give the user a raw, non-drm PDF file. If you really need to do -something-, dynamically insert a unique user ID on the last page of the book, just on the page, no steganography, write "Purchased by user #12345." PDF is pretty much a read-only format, so this wouldn't be easy to remove.

This probably won't keep people from casually sharing the book with their friends but will probably effectively put them off of sharing it publicly on the internet or p2p.

[earl]

That approach -- making paying more convenient than not -- only works for adults, ie people who have an surfeit of money and a lack of time. High school students are the opposite.

[apage43]

I'm personally fresh out of high school, still 18 in fact. If your price point is so high that your target audience will actually -look- for a pirated version before they even consider a purchase, you are not going to have very good sales. The rule of thumb for DRM is pretty much if anyone can view the content, then the content can be cracked and distributed. Any significantly good, well-known product will be available for free somewhere, it's just a question of how hard it is to find.

Consider that many high school students can't purchase things online anyway, lacking credit cards, which means no matter what, "stealing" the product is more convenient for them.