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.
On older Apple mobile devices running iOS 10 or earlier, the standard Safari browser will only download a file if the device already has an app installed that can open that type of file—e.g., using mobile Safari to download a PDF won't work on an older iPad or iPhone unless the buyer has already installed a PDF reader app such as iBooks or Adobe Reader. Newer devices running iOS 11 and later finally include a native Files app that can handle downloads of unknown file types.
That said, many devices increasingly come with some sort of download app preinstalled or can handle downloads in the browser app itself, 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.