As others have said, you create a Pottermore account, purchase the books, and when you go to "My Books" you have the option to link your Kobo account. The idea is that we then add the entry to your Kobo library, so the book becomes available to any Kobo app / device that can read your Library.
The Harry Potter books are certainly DRM free. I'm specifically looking at the iOS app and I don't see any form of encryption on the files (for obvious reasons I'm not going to say where I'm looking and what I'm looking for). There will be some digital watermarking applied, which is how Pottermore can track where any pirated copies of the book come from.