You may have noticed the password field on the checkout page. This is because the member is creating an account on your site when they purchase or sign up for a free offer. Every account has a unique library attached to it and the library will contain every product purchased from your site. Your members can reach the library by using the same login credentials created when they originally signed up on your site.
You can reach the login page by adding "/login" to the end of your site URL (example.mykajabi.com/login):