'wkhtmltopdf.exe'无法识别为内部或外部命令，可运行程序或批处理文件

Trying to generate wkhtmltopdf through html with this code.But gives me this error.

 require_once("wkhtmltopdf.php")
 $html=file_get_contents("http://www.facebook.com");
 $pdf = new WKPDF();
 $pdf->set_url($html);
 $pdf->render();
 $pdf->output(WKPDF::$PDF_EMBEDDED,‘sample.pdf’);

WKPDF program error. RESULT: 1 STDERR: 'wkhtmltopdf.exe' is not recognized as an internal or external command, operable program or batch file. STDOUT