代码点火器 - PHP - Pdf文件仅在Iphone上下载问题

I created a function in code igniter controller to return the file upon clicking download. It is working fine for every device but not on iPhone only. I couldn't understand what is the problem or what i am missing.

Only on iPhone devices it returns Garbage.

Controller method :

 public function download_document($project_id,$file_name){ 
        $doc_path=site_url('documents/projects/'.$project_id.'/'.$file_name); 
        header('Content-Description: File Transfer');
        header('Content-Type: application/pdf');
        header('Content-Disposition: attachment; 
        filename='.basename($doc_path));
        header('Content-Transfer-Encoding: binary');
        header('Expires: 0');
        header('Cache-Control: must-revalidate');
        header('Pragma: public');
        header('Content-Length: ' . filesize($doc_path));
        ob_clean();
        flush();
        $file= readfile($doc_path);

    // The PDF source is in original.pdf 
    return $file;

        }

Output on calling method