pdf download sales

Hi,

I’m sure this is a simple question. What’s the best way to sell pdf downloads? Not as in marketing, but as in technically. In other words, what software do I need so that someone clicks a button, which pays money into my paypal/google checkout and gives them the pdf straightaway in return? And is this the kind of thing I can bolt on to a normal website?

 

Thank you

 

Good to see some familiar faces!