In the footer, you can use a nested IF field like this:

{ IF { PAGE } = { NUMPAGES } "Text for the last page only" }

Note that for insertion of each pair of field delimiters, { },
you must press CTRL+F9. Once the field delimiters are in place,
you just type the instructions shown above. When you're finished,
press F9 to update the display of fields.

If you ever need to modify the text, use ALT+F9 to display field
codes, edit the quoted part, update again with F9, and, finally,
use ALT+F9 again to hide field codes from view.

Stefan Blom