These PDFs are encrypted through a hateable system, handled by the FileOpen plugin (they have a site and purchase their system to "protect" PDF). You have also to install the plugin, and the only usable viewer will be the one that can use the plugin... as far as I know, only the original Adobe Reader (and related products).
I understand, but I am absolutely adverse to, the wrong idea behind DRM (basically it's what FileOpen is about). The dangers in that idea can be discussed longly, and we also can remember this funny comics about DRM in music. But this is not the major point for FileOpen for these PDFs.
The problem here is that in order to do its job, the plugin needs to connect to somewhere, and this means that a working connection is always needed to read those PDFs! Thus the P in PDF becomes untrue, since I can't "port" those files everywhere: if there's no a working internet connection, those files become nothing but a bunch of unusable data.
Some friends of mine dream about the future: computers are terminals, the operating system being just a life support for a browser, and "in" it you do everything (cloud computing for the whole set of what a computer could be and do). This would work until there's a (fast!) connection everywhere. Otherwise the "terminal" would become really unuseful hardware only.
I hate this idea (
What is the solution? (There's no solution since we are not the one who will decide). ... But I've removed the FileOpen plugin and deleted those PDFs (unreadable data without the plugin), after all I was just curious, I can wrote algorithms by myself and I can find freely accessible literature which does not depend on the net to be read ("download once, own forever" and public libraries).
By the way, RE or so on FileOpen was done (issues about avoiding connection, key expirations... exist). But it would mean to give too much importance to them; rather, we should simply reject every FileOpen-protected ebook/PDF, in favor of freely accessible one. So they would be forced to drop that silly technology and use instead a more user-friendly (internet-independent) solution to their problems ("protecting" their stuffs).