Unfortunately I'm not the best person to ask, as I'm pretty ignorant! If you put a file in, say, a folder called /lala234looloo/, I mean, who's going to find that?! And of course, you could have folders within folders. LinkLok generates a random link that actually leads to that folder, but no one will ever know what the folder is really called.
I know that you can also place files "above the web root", though I haven't done this much, and it may be different on each host. But that's a common practice, I believe. If a file is located "above" the root folder on a server, not even the best hacker can access that, so it may be worth looking into if you REALLY need security.
Also, some other 3rd party services allow you to sell digital downloads and (I believe) host the files themselves, probably in highly secure environments. An example is eJunkie, so that's worth a look too.
For free options, I'm sure there are some around. LinkLok is a cheap, one-time fee, but if I remember rightly, services like Mal's cart offer a free cart that include digital goods (?). Maybe check that out.