It depends on how the HTTP server has been configured, which can be done (under most HTTPd's) on a per-directory level and/or globally.
If the server is configured to allow it, you can browse the contents of a given directory simply by entering it's URL into your address bar. The address of the directory is the same (usually, but let's leave URL rewriting aside) as the file's URL, sans the filename. For example,
http://www.example.com/files/Form0123.pdf is located in the directory at
http://www.example.com/files/. If the server is configured to do so it may present a
default index page when a directory with no explicit index file is requested. The
default index can, literally, be anything but is usually a listing of the directory's contents.
Browsing to a directory URL can yield any one of a number of results. First, you may get an error message, 404 or perhaps 403. In this case the server is configured to deny access to the directory
default index if no index is specified for a directory. Other configurations will yield different results and the only way to know what will happen with a given URL is to try it and see.