Readers can find the books by choosing the "Full view books" option on the Google Book Search home page before they activate their search. Once they have chosen a book from the results page, a download button is clearly visible on the top-right corner of the page.
The PDFs are offered only for those books that fall into the public domain and are intended for personal use.
"We use very conservative rules to comply with international copyright laws," Google spokeswoman Megan Lamb said.
A book's availability depends on the country from which the user is accessing the site. Google blocks users from works that are not yet in the public domain for their country, Lamb said.
A carefully worded note on usage from Google, included as the first page of each downloaded PDF file, explains what "public domain" means and how it can vary by country. Google also notes that users are responsible for following their own country's copyright laws.
"Make noncommercial use of the file. Refrain from automated querying. Maintain attribution. Keep it legal," Google lists as usage guidelines.
The bottom-right corner of every PDF book page contains a "Digitized by Google" watermark.
While Google Book Search limits the amount of copyright text a person can view in one session,, which entails scanning entire works, many protected by copyright, in order to make them searchable online. but has offered an opt-in method for publishers rather than an opt-out one.
Partners in in the United States and the United Kingdom include the University of California, Harvard University, University of Michigan, The New York Public Library, Oxford University and Stanford University.