The purchase button only displays/functions on the actual product page. It is not possible to place them on another page or website. And Exchange is a plugin for WordPress, so it won't work at all on a non-WordPress site.
I'm not sure what information you were wanting on Digital Downloads, looks like you didn't quite finish your question. If you tell me exactly what information you're needing/wanting, I can help you out.
Also, I don't know what the "PPSD" platform that your talking about it. If you explain what that is, I'll do my best to help you out.