To be perfectly honest, being a “static website system” doing “dynamic” tasks is not one of the things I would want my Hugo site to do
Having said that, the normal way for these “free for registration” goodies is generally to let people subscribe to a newsletter and then in the “hello welcome to the list” email you link to the download. You can do that quite well with Mailchimp and other email providers.
Other providers for static websites (for instance Netlify) offer authentication procedures where you can have users register themselves and load pages only when registered and logged in.
But I really think this is something that a static website is not fitting for. You wouldn’t buy a Mercedes car to play an old cassette tape you found in your attic. The Mercedes’ses cassette tape player could play it, but it’s not really what that car is for Buy a tape player.