Sharon Kimble <boudiccas@skimble.plus.com> writes:

> How can I have the footnote number in an org-mode file, exported to
> latex and then built as a PDF file, be underlined please?

Try adding the following LaTeX directive.

#+latex_header: \renewcommand{\thefootnote}{\underline{\arabic{footnote}}}

Note: both footnote number in text and in the footnote itself will be
underlined.