Call to a member function download () on null error in laravel

In function trying to get download PDF in browser while ..but i am getting error like Call to a member function download () on null error

public function index(){

$data ="<h1>Welcome</h1>";
$content= PDF::loadView('PDF.invoice', $data);
 $content->download('invoice.PDF');
}

i have tried save('invoice.PDF') and Output('invoice.PDF') instead of download('invoice.PDF')...but showing errors like member function download () on null...anyone can solve this problem!!!!

via Chebli Mohamed