This is the best "legal download" available. It is a condensed version of the 7th Edition rules that includes everything you need to play your first game.
If you decide to commit to a full campaign, owning the official Keeper Rulebook is a game-changer. The digital version is fully indexed and searchable, making it much faster to use at the table than a scanned pirate copy. call of cthulhu 7th edition keeper rulebook free download
The Keeper Rulebook is the definitive guide for the Game Master, known in this system as the Keeper of Arcane Lore. Unlike the Investigator Handbook, which focuses on player options, this volume contains the mechanical backbone of the game. Core mechanics: The D100 percentile system. This is the best "legal download" available
Chaosium Inc: Buying directly from the publisher often nets you a free PDF if you purchase the physical hardcover. DriveThruRPG: The industry standard for legal RPG PDFs. The digital version is fully indexed and searchable,
Sanity system: The iconic rules for character descent into madness. Grimoire: Over 100 spells and occult artifacts.