add_filter('frm_pdfs_dompdf_args', 'disable_remote_image_processing' );

function disable_remote_image_processing( $args ) {

	$args['enable_remote'] = false;

	return $args;

}

Use this code example to fix images from unsafe HTTPs sources that are broken in the PDF file.

add_filter('frm_pdfs_dompdf_args', 'fix_images_unsafe_https' );
function fix_images_unsafe_https( $args ) {
	if ( ! isset( $args['http_context'] ) ) {
		$args['http_context'] = array();
	}
	if ( ! isset( $args['http_context']['ssl'] ) ) {
		$args['http_context']['ssl'] = array();
	}

	$args['http_context']['ssl']['verify_peer'] = false;
	return $args;
}