To add a footer to a document, you have to go in Settings > Document and email configuration. You select the configuration in which you want to add a footer and you click on the Terms and footer tab.
Then, at the bottom of the page, you have a Footer field where you can enter all the information you want (and format it as you wish). You can also add a variable to make sure the document number is displayed on each page (which is mandatory in certain countries).
Then, click on Save.