This is the gold standard for Pathfinder players. It is the Official Pathfinder Reference Document and contains every rule, class, feat, and spell from the Core Rulebook and beyond, updated regularly with Paizo's blessing.
Rather than searching for a pirated PDF, you can use these officially sanctioned tools to access the same content: pathfinder core rulebook pdf free download
A modern digital toolset that offers the Pathfinder Primer for free. This "mini" core rulebook covers the basics of play in a sleek, mobile-friendly format. This is the gold standard for Pathfinder players
While not a book, this web and mobile app is an essential character builder that includes all rules for classes and ancestries, making it easier for new players to learn by doing. Why the PDF Is Worth Buying mobile-friendly format. While not a book