Selling downloadable PDFs - What's the best way to set this up?

[mayonnaise18]

Hello!  My client has authorize.net and is planning on selling 1 downloadable PDF.  It'd be preferable that the customer can automatically download the PDF right after checkout and the link changes for each customer so that they can't share the file link later.  

I see I can do a simple buy button but my client would have to send the file each time and the customer would have to wait for the file.

What do you recommend for this set up?  Is there a third party we should use?  

[TJPride]

You can't do this with Simple Checkout. You could, however, switch to DPM, which allows you to update your database as the transaction goes through and serve the file immediately on completion.