The direct source is always the best place to start. Paizo offers its entire legacy catalog of First Edition books in PDF format. When you buy directly from Paizo, your PDFs are watermarked with your account details and stored in your digital downloads library forever, allowing you to re-download them whenever needed. 2. DriveThruRPG

If you only download (legally purchase) five PDFs to run a campaign, make them these:

: Massive expansions to spells, feats, and combat styles.

Pathfinder 1e is notorious for complex rules interactions. A digital PDF allows you to use Ctrl + F to instantly look up specific feats, spells, or conditions mid-game.