Apache is pretty useful for sharing files on network particularly on LAN. Here is how to map an external folder to an url on your apache server. Lets assume your user name as bob and the path of the folder that you want to share is /home/bob/downloads. And you want to access contents of this folder at url /downloads.
Change directory to /etc/apache2/sites-enabled. And create file named downloads with the following contents.
Alias "/downloads" "/home/bob/downloads" <Directory "/home/bob/downloads"> AllowOverride None Options Indexes Order allow,deny Allow from all </Directory>
Save the file and close it. Now restart the apache by issuing following command.
sudo apache2ctl restart
Now test everything is working as expected by going to following url in your browser.