A Word document

This document could have been created by your client in Microsoft Word and uploaded to the server by them. As long as they always name the document the same, then it will always be the content of the iframe.

Modern ftp programs like CuteFtp have Windows Explorer like interfaces so that, for your client, it can be simply a matter of dragging the file from their local machine to the server similarly to how they would drag a file from one folder to another on their local machine.