For our merchants selling e-books in PDF format, we have been working on a couple of ways to protect their PDFs from distribution.
We tried the DRM angle, but it was really making the buyer experience poor, was not supported by all the PDF readers and it was apparent that in various scenarios, it can cause additional customer support issues for the merchants to handle. So, we decided to scrap that.
And then we worked on the PDF stamping feature .. basically, we can now stamp the buyer's name, e-mail and the transaction ID on the top-left of each page of the PDF before the download. Unlike DRM, this is a simple, non-intrusive way to discourage buyers from sharing their PDFs.
EDIT: Please see current documentation for PDF Stamping in our help pages here: