Password Protect Files

I've just paid to have an FTP site set up. I'm able to navigate through the site, create folders and download data using SmartFTP. What I can't figure out is how to password protect the files I set up. I'm in the printing business and want my customers to be able to download their files to my site, preferably into a pre-assigned folder that only they can get into. Any help would be appreciated.

Thanks,

The company who runs the FTP server will need to create a user group
account, with limited permissions. Such as to view and download only.