Sure, you can do it, I don't see Bruce P. or his publishers following you around in a black van waiting for you to let your guard down, or should I say "break the tension", if you'll pardon the pun.
The real question is => If roles were reversed, you were the author of a good chess book and somebody else did what you did, would you be cool with it? What if 1000s did it? What if your publisher said that your numbers are low because people are not buying it anymore although you know that over a gajillion people downloaded the PDF (because they can) and are reading it on their iPad (bought ironically with money that should have also gone to you)?
If this is a non-out-of-print book you can buy (even used) and you actually plan on enjoying it cover to cover (not just skimming), I'd say you owe it to chess literature in general to buy it and make sure the author is rewarded for helping you.
There's no way to rationalize this being okay. If it was open season on downloading chess books which you can still buy, you're going to start seeing the watering down of good chess literature the way popular music reached the gutter during the past 10 years.
Recently I found some different e-books, pdf, and pgn files on chess. I thought I had hit the jackpot completely. For example: in chesszone.org I was very much attracted to Pandolfini's Endgame Course.
So is it allowable to download it and use it? Or am I like destroying the chess knowledge market? Does this apply to all Chess books (say I found more recent books, like "Reasses Your Chess" by Silman, or "Dvoretsky's Endgame Manual")?
Any help would be much appreciated.