Prolific Works used to be InstaFreebie. They are a service authors use to pass out free books in exchange for newsletter subscriptions. Some authors make subscribing mandatory to receive the free book, others make it an option. If you see a Prolific Works link, it is a legitimate thing (I wouldn’t allow it on here otherwise, as I create each landing page, myself).
Also, you won’t know if the subscription is mandatory or optional until you click on the link. It will let you download the book and then ask you if you want to hear more about that author. Click yes to get on their newsletter, click no to stay off their list.