How can I add a footer text to a theme?

There is no support for adding a footer text in the theme edit screen directly. But you can add a background image with a footer text in the image itself. 
There you can find some inspiration: 11 inspirations for footer text

You can upload your images with a footer 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?

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