Download files from a Web site.

Tenders Web site which contains information about the projects for construction industry. The registered users divided by different trades: architects, builders, plumbers etc. For different trades may be different files (drawings, photos) to be downloaded, so I need to allow for the Builders to download only necessary files and restrict the other trades to download the Builders files. I will put the links for those files at the aspx page, if the user logged as Builder (Plumber, …), so they will download them.
But if the address of a file will be known to somebody, he can download it as well.
Question: how can I permit only for the designated person to download the files?
Thanks for any suggestions.

It sounds like you need to consider Authentication and
Authorization. You need to verify someone's identity and
then grant access only to authorized resources.

Your architecture will need to be designed around these
core components.

One option is to model the security access in the DB so that
the user only sees what they are allowed to upon logging in.
But supplement this by implementing file-level security so they
truly can't get at anything naughty. If you do this via Group
membership, it makes it a bit easier for admins to manage.

Thanks fungi for your suggestion, I think I will need to set a <location> tag in the Web.config file and set permissions for each directory for the groups of the users. And those directories will represent the trades: Builders, Plumbers, etc. So files for Builders trade will be stored n the Builders folder, etc.