Once you have an account with the portal you may have the ability to send documents either in the form of a request, or at will via your portal main page (after logging in).
If it is in the form of a request you will see something similar to the document signing screen.
However, in place of the sign and reject buttons you will see the reply button:
To reply to the document simply click the reply button at the top of the document.
This will show a screen giving allowing you to choose a document from your computer and, if permitted by the publishing company, an area to add an optional message:
You can select 5 documents at a time and up to 100MB per document. Once you have added your documents click send.
You will then be presented with the following confirmation:
You can also send a document at any time via the portal main page.
This is located under the COMPOSE box: