If you want to use a URL then I would use a URL field, or potentially a text field so that I can allow relative URLS if that what you’re looking for.
But this will not work if you want the person to be able to upload a file because ACF uses the WP media uploader for files.
There are ways in WP, filters, that would allow you to specify a custom path where a file should be saved but there really isn’t any way to get around the way the WP handles files as “posts” using ACF.
