copyright owner.

As an author I have the option of applying ADE to the PDF files I make, and thus the person reading it needs a certain program and code to read it, but they can then read it on any device they enter the code into - this is a form of DRM managed by the copyright owner. While the stopping of a Kindle book being read on a non-Kindle device is a restriction by the hardware seller, Nook, and Apple do the same.