You can buy the products you have selected everywhere, also on eCommerce websites. Please check Terms & Conditions.
No, it isn't.
No, each voucher can be redeemed just for the quantity showed on the voucher. Controls are made in order to avoid frauds.
You can print the voucher just for your documentation. You'll find on this site the list of products you have selected, if still valid.
Just photos of receipt or invoice taken by a camera are accepted, even if the document has been issued in pdf by the vendor.