If you activate the option "Settings > Configuration > General Settings > Portal access > Activate the customer portal", what will happen is that every email notification sent by OpenChatter will have a link at the bottom of the email that points to the documents in your OpenERP instance. It will allow any follower (internal user or external contact) to see the document.

On that topic, I also recommend to read: https://accounts.openerp.com/forum/Help-1/question/69/