When you post a file or create a folder on a Content Area (e.g. the Content or Information pages), you have an option to make it "Unavailable". This removes the file from what students see on the content page, but does not prevent students from accessing the file via other methods. Thus, making files "Unavailable" should only be used as a convenience to help students focus on files that are relevant at a particular time or under particular circumstances.
That said, Ted does allow you to set permissions on a file so that students cannot access it.