- Site A generates a Twit button code like [url removed, login to view] for everyone.
- Site B publishes the code copied from site A.
- Vistor C clicks the button on Site B and twits the predefined text (maybe with a link).
- When his twit is noticed by the app the download button under the twit button () becomes active (twit and download).
Download button can be linked to a PDF or a MP3.
Also statistics are requirred.
Other than those if a Twitter user gives necessary permissions to the app then the app will be able to follow anyone for him/her.
Can you do all of these, if yes how long will it take and how much do you want? I just filed budget and time boxes because they were mandatory, they are up to you.