Mobile devices like Android or Apple tablets and phones do not have, like their desktop or laptop brethren, a traditional file system to download files to. This means that when your customers attempt to download a file, their device will always try to open it in an appropriate app. If there isn't one, it won't know what to do with the file.
Our recommendation is to encourage users to download files on a desktop or laptop computer. You can use the "save to dropbox" feature on mobile devices to have users download the files directly to their Dropbox account, and bypass their local lack of filesystem support.