it’s a bug, after this line in the custom_report script, insert these 4 lines
$config_mpdf['margin_top'] = $config_mpdf['margin_top'] * 1.5;
$config_mpdf['margin_bottom'] = $config_mpdf['margin_bottom'] * 1.5;
$config_mpdf['margin_header'] = $GLOBALS['pdf_top_margin'];
$config_mpdf['margin_footer'] = $GLOBALS['pdf_bottom_margin'];
will get this added to the next patch