There is no support for adding footer text in the theme edit screen directly. But you can add a background image with a footer text in the image itself.
Here you can find some inspiration: 11 inspirations for footer text
You can upload your images with footer text to the background of the Cover page and the Inner page.
You need to edit your theme / Cover page or Inner page
How can I customize proposal background image?
You can add page numbers to a proposal PDF too: How do I add page numbers to a proposal PDF?