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.
And do you have a link you can send me for more information on how the digital downloads.
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.