When you buy the PDF from Chaosium’s own store or DriveThruRPG, you get access to the official forums, fan content, and—crucially—the where creators like Mike Mason answer rules questions.
The two most famous Pulp Cthulhu campaigns are: Pulp Cthulhu Pdfcoffee
Sign up for Chaosium’s newsletter. They run a “Christmas in July” sale where Pulp Cthulhu drops to $15. When you buy the PDF from Chaosium’s own
Pulp Cthulhu is a supplement for the Call of Cthulhu 7th Edition Pulp Cthulhu Pdfcoffee