Gumroad - where I self-publish my book (https://gum.co/deploymentfromscratch https://gum.co/deploymentfromscratch) - does the watermark automatically for PDFs.

Gumroad - where I self-publish my book (https://gum.co/deploymentfromscratch https://gum.co/deploymentfromscratch) - does the watermark automatically for PDFs. That said, probably don't worry about it. I also release additional things (like code), and it's just there for anybody that bought it. I am fine with that. It would take you a significant effort to prevent bad actors and you would likely make it worse for the your customers in the end. It's just something to accept.