str_contains( $url, '#' ) ) {
		return false;
	}

	// If it doesn't have a PDF extension, it's not safe.
	if ( ! str_ends_with( $url, '.pdf' ) ) {
		return false;
	}

	// If the URL host matches the current site's media URL, it's safe.
	$upload_info = wp_upload_dir( null, false );
	$parsed_url  = wp_parse_url( $upload_info['url'] );
	$upload_host = isset( $parsed_url['host'] ) ? $parsed_url['host'] : '';
	$upload_port = isset( $parsed_url['port'] ) ? ':' . $parsed_url['port'] : '';

	if ( str_starts_with( $url, "http://$upload_host$upload_port/" )
		|| str_starts_with( $url, "https://$upload_host$upload_port/" )
	) {
		return true;
	}

	return false;
}