Google Books is a service designed by the search engine that allows you to find books from all around the world and even download them in PDF format or just view images with its covers. The service is extremely useful for a lot of students from all around the world because it helps them to find their literature with a simple and powerful search engine. Because the service offers free PDFs download, Google gets all the copyrights for the documents as no owner or author is mentioned. That's why Google decided to impose some restrictions to the PDF files and restrict them for personal use.

"Make non-commercial use of the files ... we request that you use these files for personal, non-commercial purposes. ...
Maintain attribution ... Please do not remove [the Google "watermark"]," some Google limitations are saying.

"The Google Books project has seen a lot of criticism raised at it, especially when the project attempted to make digital copies of copyrighted material - without the permission of the copyright holders. Now the allegation is that Google are trying to impose copyright restrictions on copyright-free books they produce - suggesting that Google really doesn't have a clue about copyright issues. The claim states that Google are seeking to publish the following restrictions in their guidelines for PDF's created for the Google Books project," Platinax said.

Personally, I think that Google should allow unlimited use for the PDF documents because the company doesn't have any right for owning or distributing the files. As Google receives the documents from other companies, organizations or libraries, the giant search engine should give full credentials to these firms and let clients use the files as long as these companies are allowing.