How to avoid URL direct access?

Hi:

I have a documents program in mi intranet, it only shows each user
authorized files and each line has the file description and a link to
open it.

The problem is that if anyone writes the file's URL address directly
into the browser (like: http://mydomain/myintranet/myfiles/myfile.pdf)
the file is opened without pass through the Intranet security.

How can I avoid the direct file access?

Regards,
Gus

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