Going to buy ebooks for the first time. Question on where to read it.

ebooks, epub, IFTTT, pdf, reddit,

Hi guys, I'm planning on buying e-books at Rakuten Kobo, after I buy them what happens? Do I need to download a specific app? I'm planning to read on my phone and desktop computer, are the files transferrable? Thank you! (sorry for the newbie questions) via /r/ebooks https://ift.tt/NkUXDZo