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 directly from the Web.
Unfortunately, Web-based downloads to mobile devices (e.g., smartphones or tablets, including iPads/iPhones) can be a bit of a mixed bag, depending on the particular device and what software the buyer has installed on it; 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. Buyers should at least be able to download their file to a regular computer and then sync the file to their device from there.
The standard Safari browser on Apple iOS devices in particular will only download a file if the device already has an app installed that can open that type of file -- e.g., using Safari to download a PDF to an iPad or iPhone won't work unless the buyer has already installed a PDF reader app such as iBooks or Adobe Reader.
That said, many devices come with some sort of download app preinstalled, and most users who want to download things to their device from the Web and have done so previously would already have sorted this out on their own, so it's really only an issue with buyers who have never downloaded anything to their device from anywhere but the device's official "app store" before.
If you expect to have a lot of mobile buyers, you may wish to customize your thank-you/download page with download tips for those buyers in case they have trouble downloading. This discussion in our forum covers this topic in more detail.