This is why I like places like Humble Bundle, you literally get a download link with PDFs (as well as other formats). It encourages piracy of course, but then again the moment somebody else could view your content on a device you don't control - it was already copy-able.

If the ebook publishers were motivated to, they would encode unique properties into the text of each download that uniquely identifies the transaction. Then if you find copies online you know who to chase.

The point is, give me my copy of the file, I don't want to be locked into your system.