How to set up secure download page

I’m new at WordPress and php, but am starting to understand some of it.

I’m trying to set up a button for Paypal, upon a payment in Paypal I want a redirect to a page where the customer can download an e-book.

WP Paypal Shopping Cart does most of this, but as far as I can tell, there’s not a way to go to a secure page with the download. In other words, if someone manages to type in the name of the page that I have set up for the download page, they get a free book.

Is there either a plugin that I could use to secure this page?
Or is there maybe a way that I can make the page only accessible from the Paypal purchase page?