If this is a user customizable portal (which it appears to be) then you could create a new dashboard widget that contains the link and then add it to this slot in the portal as described in the article below:
I agree with Marc. You have to create a section which will be treated like a dashboard widget(see settings tab of section rule) and and the widget can be added to the portal by a settings icon present in the user portal.
You can use the link control(check for basic drop down menu in section rule) in the section and refer URLs to the documents. It will open in the browser or may get downloaded depending upon the URL you are giving.