As John mentioned, this is simply security through obscurity. What that means is it's basically not security at all.
Once somebody knows the URL for one of the files there is nothing you can do to stop them sharing it and giving it to other people. Then anyone can download it and you can't stop them.
Also, if somebody downloads several files, they may be able to determine a naming convention. Perhaps your files are called file1, file2, file3 etc... Once someone learns there is a pattern they could enumerate the rest of your files.
Failing that, unless your file names are incredibly random, someone could simply write a script to brute force guess your file names.
The point is, if you need them to be secure, make them secure. By hiding them (obscuring them) you are not making them secure. Obscurity is not security.