Well, of course I'd encourage you to have your friends buy copies too, I can see how convenience and cost might not balance out well.
For instance, I own a copy of Sword & Fist, and one of my players plays a Fighter, so he asks to borrow S&F for a week. So the book is at his house. Then, one night I'm prepping for a game, and I realize that I really want a Lasher as an NPC. Now, I could call my friend at 3am and ask him to bring back the book. Even if the NPC was really important, I probably would not do that. However, if I had a pdf of S&F (y'know, let's assume WotC sold the pdf), and my friend asked to read it, I would feel safe letting them borrow my printed out version, since I have a copy on my computer. With a normal book, I'd just have to grin and bear it, and wouldn't decide to go out and buy a copy of S&F at 3am. So, in this case, I don't think the publisher loses out.
Now, if two friends both asked to borrow the book at the same time, with a normal print book I could only let one borrow it. Realistically, player 2 would just wait for player 1 to finish, probably. If he really liked the book, he'd buy a copy himself though. If I had a pdf, though, I might think, "Okay, give player 1 a copy, and player 2 a copy, and keep the pdf file for myself." In this case, the publisher might have lost out on a sale.
Finally, say you want all of your players to have copies of the rules for, say, Bahly (a game from Tournaments, Fairs, & Taverns). You could make them all buy the pdf for $5, but that's not fair. Instead, you just print 4 copies of the three pages that pertain to Bahly. That's why pdfs are cool, since you can do things like this.
So, in general, I'd encourage you to only print one copy at a time. In a pinch you can still read it yourself on your computer, and it's quite unlikely that you'll cheat the publisher out of a copy. Of course, if your friend says, "Yeah, I'm really thinking of buying this book, but I want to take a look first," sure, print him a copy if he wants, but hold him to buying a copy if he likes it.
Basically, it's a not a clear-cut issue. I'd say that you can definitely make partial copies for whoever you want, if those copies pertain to the game. At most, though, you shouldn't print more than 2 copies (one for yourself, one to let someone borrow).