It’s far better to use a plugin such as Easy Digital Downloads as there are many steps you will need to code in e.g.
1) Check if user has paid (by checking if they were referred back successfully by Paypal) and then set the option in the admin area to say yes you have paid
2) Check what you have bought and direct you to download the item
3) Set a limit on number of downloads of product
4) Stop anyone accessing the download page without a download token
Far easier to use one of the many plugins that have been built as there is a lot of work involved in building a custom one which it’s just not worth doing unless you are trying to build something unique.