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.
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.
Apple iOS devices
The way file storage works on Apple iOS devices is inherently kinda weird and not what many people are used to. iOS has no general-purpose file storage, like the drive/folder/file scheme we're accustomed to from regular computers. Rather, each app in iOS has its own file storage, and only for file types that particular app can open. You and your buyers may find this Apple help page informative for performing downloads to an Apple iOS device.
In a nutshell, when a buyer taps our Download button on their iPhone/iPad, they may then need to select which app on their phone can open that type of file, then the file will be stored in that app. Or, if their browser opens the download for viewing in the browser automatically (e.g., as may happen with PDFs and common audio/video files), the file will remain stored in their browser app's Downloads list—in which case, they can just bring it up from there later when they want to view it again, or they can "share" the file from there to another video-player app they have installed.
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.