When you buy a book you will first be sent to PayPal, where you can choose to pay with either a card, like Mastercard or Visa, or you can choose to pay withe PayPals own system. Either way you get an email with a download link.
Do remember that the link is only valid for 24 hours!
Many of my customers who use Windows (I know nothing about Andoid), have problems with zip files.
Mac and iPad automatically open such files.
I can recommend programs like Izarc and WinZip for Windows users.