Show footer on PDF only on the last page

Found out how.

https://wp.dinamiko.com/demos/dkpdf/doc/filters/

function custom_dkpdf_after_content() {
	$output = '<div style="margin-top:25px;width:100%;background:#CCC;">After content text</div>';
	return $output;
}
add_filter( 'dkpdf_after_content', 'custom_dkpdf_after_content' );

Refer to comment above.

Hello Michael, I know this question is sort of silly, but where (that is into which .php file(s)) do I insert the function and filter to make your solution work? Thanks!