I've thought of this before and here is what I came up with. Each ebook purchased should be unique. This does not have to be a manual process. You should take the transaction ID of the ebook purchase, use it to generate a steganographic image, and patch that somewhere into your PDF file. A good way to do this might be to use Least Significant Bit encryption. Directly quoted from Wikipedia:
>>>For example: a 24-bit bitmap will have 8 bits representing each of the three color values (red, green, and blue) at each pixel. If we consider just the blue there will be 28 different values of blue. The difference between 11111111 and 11111110 in the value for blue intensity is likely to be undetectable by the human eye. Therefore, the least significant bit can be used (more or less undetectably) for something else other than color information. If we do it with the green and the red as well we can get one letter of ASCII text for every three pixels
Now, there are two main problems here, as I see:
1. The text can still be scraped from the ebook and redistributed as a .doc file. For this reason, you may also want to include some text steganography (e.g., draw some ASCII art with varying characters; change the order of people you thank on the "dedication" page).
2. The pirate-distributor could insert pixel noise randomly all over the file. Then, your trans-id information encrypted in the LSB-image would be lost. In my opinion, however, this is unlikely, because the security I've described is not (at least typically) used in practice (with eBooks, as far as I know).
These aren't preventative measures [1]. But if something does end up circulating on the net, you have a unique ID and can then probably sue their parents. Be sure to explicitly forbid them from circulating the PDF/consent to being sued if they do/what have you, in the terms of service.
[1] Unless you explain that each ebook is uniquely secured, which may prove something of a deterrent; unfortunately, one measure of security is to not let people know that the property is secured in the first place. This may make the steganography more vulnerable to attack.