Humble Bundle runs a “Pathfinder 1e” bundle roughly once every 18 months. For $25-35, you can get 40+ PDFs, including the core rulebook, bestiaries, adventure paths, and player companions.
This is the primary source. When you buy a PDF from Paizo, it is tied to your account forever. You can re-download it any time, and Paizo frequently runs sales where books drop to $9.99 or less (down from standard $19.99).
The primary appeal of Pathfinder 1E lies in its sheer volume of content. Unlike newer systems that prioritize streamlined mechanics, 1E is celebrated for its complexity and the "breadth of content" that allows players to manage almost any imaginable in-game situation. This depth is preserved in digital libraries that house hundreds of different products, including:
: Foundations like the Core Rulebook and Advanced Player's Guide .