Unfortunately, Web-based downloads directly to mobile devices, like smartphones or tablets, can be a bit of a mixed bag, depending on the particular device and what software the buyer has installed on it -- i.e., there's no way we can force a device to download something from us if it has no such download capability built-in or installed, nor can we dictate how the device handles any downloaded file. This discussion in our forum covers the topic in more detail:
Buyers should normally have no trouble downloading from our system to an actual computer (including laptops and netbooks), and they should be able to sync the file to any mobile device from there. If your download link works on a regular computer, it should work just as well on any mobile device that's actually capable of downloading from the Web.
If you expect to have a lot of mobile buyers, you may wish to customize your thank-you/download page with instructions for those buyers in case they have trouble downloading; this help page explains thank-you page customization: