If you consider yourself an art lover, a student, or just someone looking to add a touch of culture to your coffee table, you have likely heard of "The Art Book" by Phaidon.
The New Edition: Updated to include more contemporary artists and diverse voices that were missing from earlier prints. The Value of the Physical Copy
The book is designed as a user-friendly, non-traditional visual sourcebook. The Art Book: Editors of Phaidon Press: 9780714844879
The Internet Archive (archive.org) often has "borrowable" versions of out-of-print editions of The Art Book. You cannot download the file permanently, but you can read it in your browser for a set number of hours. This is the closest to a "free" legal PDF you will get.
Standard Edition: Large-scale format for clear visual detail.