Hello, I've been using itch.io as a developer for a while but I'm about to switch to a slightly different type of project and I need some information on how exactly itch.io's download keys work.

I'm planning to publish a book, both in physical and pdf form, and I want all the digital downloads to be available on itch.io as its own project. Additionally, everyone who buys a physical copy would gain access to the digital version for free. I've been planning to include a download key as a QR code inside the physical copy of the book, but I'm uncertain about how exactly that would work. Can every download key only be used once? If so, I would need to print them separately from the book and distribute individually, one for each copy, which would make a big difference in planning out the whole digital integration. Also, if the same download key was used by several people, would itch.io keep track of how many copies were claimed and by who, to avoid possible exploits and link sharing?

TL,DR: I'm lost on how download keys work and if the same one can be used to grant access to multiple people, help appreciated.

You decide if download keys can be claimed or not when you create them. Claimable keys can be used only by one person, who then gets ownership of the product as if they had bought it. Sounds like this is what you want. The other kind can be used repeatedly, and/or by multiple people, but can be canceled at any time if you suspect people are abusing them. Hope this helps.