public function fillForm($data, $encoding = 'UTF-8', $dropXfa = true, $format = 'xfdf')
    {
        $this->constrainSingleFile();
        if (is_array($data)) {
            $className = '\mikehaertl\pdftk\\'. ($format==='xfdf' ? 'XfdfFile' : 'FdfFile');
            $data = new $className($data, null, null, null, $encoding);
        }
        $this->getCommand()
            ->setOperation('fill_form')
            ->setOperationArgument($data, true);

        if ($dropXfa) {
            $this->dropXfa();
        }
        return $this;
    }