A large problem almost everyone who sells files online has to deal with is the threat of someone leaking your thank you page or download link. In fact, you can pretty much kill your chances of selling a product if a forum post with your unprotected download link is just a Google away... Instead, you have a couple of options when protecting your downloads and we'll cover them all from worst to best.
This one is decidedly low-tech. You simply adjust the URL to the thank you page and download page (as well as the link to the file) and make sure to send out the new URL form now on. This works fairly well except its a pain in the butt and it doesn't fix the hole, it just moves it! However, this is most definitely the easiest to set up (even though its the hardest to maintain!).
This means the file doesn't even sit on a URL and only exists on your own personal hard drive. When someone buys your digital product, you simply attach the file to an email and toss it off. This works great if you don't have many sales, the file is really small, you have lots of time to personally email each customer, and don't have customers that demand instant access. However, if even one of those requirements aren't met, keep looking!
This one is actually rather clever, as long as you know how to program. Create a script that gathers your PayPal IPN and sends off an email containing a custom, one time use URL for downloading the file. Don't forget to not link directly to the file but to route it through the script in memory (and deny the download if the IPN or customer URL wasn't valid). Not the easiest in the world, but it works!
This one is my favorite (how original of me). I've designed BitBuffet to be simple and easy to use. Just set up your PayPal account and upload your file. That's it! It's like we created a script for you and then hosted they file for you as well. Never worry about missing an email as that's done automatically and instantly, and never worry about changing an URLs or someone sharing the download link.
Not sure if you really want to use BitBuffet? Why not just sign up for a free trial and see if you like it? You don't have to pay a dime upfront and just pack up and leave if you don't like us (though I know you will!).