protected function get_document_converter_classes($from, $to) {
        $classes = [];

        $converters = $this->get_enabled_plugins();
        foreach ($converters as $plugin => $classname) {
            if (!class_exists($classname)) {
                continue;
            }

            if (!$classname::are_requirements_met()) {
                continue;
            }

            if ($classname::supports($from, $to)) {
                $classes[] = $classname;
            }
        }

        return $classes;
    }