Here's one way to do it:
Create a directory called public_html under your home directory, and put the file there.
Start up apache
(you may need to give the whole path to apachectl; use “locate apachectl” to find it). (You may also need to install apache from your distro's install CDs or http://www.rpmfind.net/
Now tell them to download http://IP
is the IP address of your computer, user
is your login username, and file
is the name of the file.
Make sure that your home directory and public_html are readable and executable by user nobody, and that you don't have port 80 blocked by a firewall.