To add a footer to a document, you have to go in Settings > Document and email configuration. Select the document configuration in which you want to add a footer and click on the Terms and footer tab. Then, choose the document for which you would like to add a footer:
In the Footer field, 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.