"This is a digital copy of a book that was preserved for generations on library shelves before it was carefully scanned by Google as part of a project to make the world’s books discoverable online.
"It has survived long enough for the copyright to expire and the book to enter the public domain. ... Public domain books are our gateways to the past, representing a wealth of history, culture and knowledge that’s often difficult to discover. Marks, notations and other marginalia present in the original volume will appear in this file - a reminder of this book’s long journey from the publisher to a library and finally to you."
It's really very simple, if you're looking at a public-domain book, a download button loads with the page and tells you how large the file is. Just click it and the PDF is downloaded. For an example, check out Pride and Prejudice or Origin of Species, depending on your tastes.
Unfortunately, these are just scanned images, not text. If you read any reports that these files would be instantly searcheable (as I did) those reports were false. However, the books are instantly searcheable online, proving that Google does indeed have the ability to read and convert these documents into text. Let's hope that's next on the agenda.