You don't really want to read ebooks as PDFs, unless you have a Kindle DX or are reading on a computer screen. PDFs aren't reflowable ... the lines of text don't rewrap to fit the display. You have to scroll this way and that to read. Also, since the pages are images, the files are large. Usually over a megabyte.
You may have to do this if you're a specialist or researcher using Google Books to access old, out-of-copyright books. But it's not the best for pleasure reading.
It's much better to get your ebooks in reflowable text formats like Mobipocket, Kindle, or epub. Epub is the emerging standard. You can download the books at Project Gutenberg or Manybooks.net. You can also find books that are out of copyright in Australia (but not the US) at Project Gutenberg Australia.
Manybooks serves up books in many formats. At this moment, they offer 25,965 free books.
You can read ebooks on a PalmOS PDA, on an iPhone or Blackberry, on many phones, on netbooks, on Kindles, Sony Readers, and on the just-released Apple tablet.
I'm one of the people who make the free text-format ebooks. I volunteer at Distributed Proofreaders.