Cheaper Than Used Amazon Technical Books

I was searching for a technical book the other day. Normally I just go to Amazon and buy whatever I'm looking for there but on this day I did a Google search. After having found the title on Amazon already a site that also had the title caught my eye. The name of the site was "Computerbooks.web.com" and the title of the main page, which was "Free Computer Books, Tutorials & lecture notes," made me stick around.
I buy a lot of technical books in any given year. At $30 – $60 USD a book the amount of money I spend can add up quickly. The books I'm finding on this site all seem to be in a PDF format that you need to read off a computer screen. I've heard that the average person reads the printed page about 30% faster than reading off the screen. I'm sure I will continue to buy books and read them off the printed page but if I can try before I buy or only read a section I need online I may save a few dollars here and there.