There are a few options that make it easier for readers to find free PDFs: Google Books (gives access to books in the public domain in various fields, both national and foreign). Digital Library of Rare Works USP (USP's digital collection that makes academic life easier). Open Iibrary (a site that aims to catalog all existing books and already has more than 1 million works ready for download) and the Domínio Público Portal (a site that allows you to search for books in PDF for free).