If you cannot find a legal PDF, or you want to support the preservation of classic texts, here are your best options:
The textbook " Thermodynamics, Kinetic Theory, and Statistical Thermodynamics
Why choose Sears & Salinger over a modern book like Thermodynamics: An Engineering Approach by Cengel & Boles?
The most widely referenced PDF is the (published 1975, Addison-Wesley).