Here is a quick screenshot demonstration of how does the purchase process appear to the end-user. Take a look at the Quick Start Guide and Backend Screenshots as well.

After clicking on PayPal button user is redirected to PayPal to complete the payment ...

When the payment is completed user is redirected back to your page where the download link appears.

If the user registers on your website with his PayPal email address, he will be able to re-download the file from the Files Page.