将pdf文件列表保存到zip并下载

I'm want to download a list of file pdf into an zip file. But when i download it the file pdf inside the zip have size 0kb. i will post the code below.

    $this->load->library('zip');

    foreach ($candidati as $candidato)
    {
        if($candidato->CV_candidato){
            $file = $nome . '/' . $candidato->CV_candidato;
            $this->zip->add_data($file);
        }
    }
    $this->zip->download($nome . '.zip');

Some suggestion? Thank you for your time.