I prefer to have the physical copy of the book, myself. There's just something satisfying in flipping through pages and perusing them until you find what you're looking for. But, that said, pdfs are sometimes quite handy, because it is far easier to carry a laptop vs a stack of books. And I don't believe it's piracy if you're DLing a pdf of a book you already own in hardcopy.
DLing books you don't already own, I personally don't have a problem with it. Everyone has their reasons for doing it, whether they're good reasons or not, and if they aren't used, no harm done. If they are being used, then hopefully someone they play with does go out and buy a copy, which will in turn indirectly help the game developer.
This thread reminded me of one conversation I had about all the game books I buy. Someone had asked me how I can justify spending $40 on average per book. I looked at her and said, "I buy one to two books per month, plus other gaming stuff, and I don't smoke, drink, or do drugs. I probably spend an average of $100 per month on it. But a year from now, I'll still have them and still likely be using them. You smoke a half a pack a day. That costs about $5 a day (we live in Canada). So you spend about $150 a month on cigarettes, and don't have anything to show for it. Who has to do the justifying here?"
She never bugged me about my gaming habits again.