Best way to offer a paid download on my WP site?

I plan on selling an informational product (PDF). I’ll use paypal to accept payments, but then I need to redirect the users back to my site.

I’m trying to figure out the best way to set up a download page on my blog but make it private or password protected or something, so it’s not just a link that anyone can type in and get the product for free!

If there’s an easy way I’d love to know.

If not I’ll use a third party vendor like ejunkie