Use this code example to add rel="" to the [frm-pdf] shortcode, e.g.
[frm-pdf id=x rel="noreferrer"]

add_filter('frm_pdfs_default_shortcode_atts', 'filter_default_atts');
function filter_default_atts( $atts ) {
	$atts['rel'] = 'nofollow'; // Default is rel="nofollow". 
	return $atts;
}

add_filter('frm_pdfs_shortcode_output', 'filter_output_shortcode', 10, 2);
function filter_output_shortcode( $output, $atts ) {
	if ( ! empty( $atts['label'] ) ) {
		$output = str_replace( '<a', '<a rel="' . esc_attr( $atts['rel'] ) . '"', $output );
	}
	return $output;
}