The PDFs for the Pathfinder books from Paizo had a problem that limited their utility, though. They have multiple layers with lots of vector art. This is great for getting the best image on any display or from any printer. It's a huge load of computation, though, if you just want to read the rule book on an e-reader or a laptop.
I originally bought the PDF of the Core Rulebook to be able to cut and paste items into my own adventures--little reminders for rules that I may not use often that would crop up, and other such things. The PDF did that job great. But when I went to put it on my Sony PRS-950 e-reader, so as to have a small, light copy of the rules where ever I went, it buried the poor e-reader's processor. Page turns took forever. It was nothing but frustration.
Even the version of the rule book that has the chapters as separate files didn't help. The files are smaller, but the computational overhead was still just too much for my e-reader. Even when I threw it on my Eee PC (a "netbook" style laptop computer), it was just too slow to use. I was reduced to extracting the text from the PDF to rather ugly text files.
Fortunately, Paizo has now released Lite PDFs of the Pathfinder books.
I downloaded them this weekend, then tried them out on my e-reader and Eee PCs this morning. What a difference! They read smoothly and well, even on the Sony PRS-950's little processor (little by current standards--it's got far more power than my primary software development workstation from the '90s!)
I'm not the only one loving these new streamlined PDFs. Have a look:
The Iron Tavern Mini Review: Pathfinder Lite PDFs
The Earthen Ring comments on Pathfinder Lite PDFs
Paizo Messageboards: PFRPG Lite PDFs