With the Joomla! extension "FTP System", it gets very easy to share documents with your clients or friends. You can upload and download files when you have an account. You can try the demo at:

Username: demo
Password: demo

When you logged in and goes to "Configuration" you will see two types: "Type 1" and "Type 2". When "Type 1" is enabled the download & uploads are private and with "Type 2" is enabled you can share the uploads & downloads with all the users.

Have fun!