Say I have a website www.example.com. Under the website directory there is a page secret.html. It can be accessed directly like www.example.com/secret.html, but there are no pages that link to it. Is it possible to discover this page, or will it remain hidden from outside world?
If you have directory listing disabled in your webserver, then the only way somebody will find it is by guessing or by finding a link to it.
That said, I've seen hacking scripts attempt to "guess" a whole bunch of these common names. secret.html would probably be in such a guess list.
The more reasonable solution is to restrict access using a username/password via a htaccess file (for apache) or the equivalent setting for whatever webserver you're using.