how to fix Class 'PDF' not found in laravel

enconted following error in My Laravel app

FatalErrorException in CollaboPDFController.php line 14:
 Class 'PDF' not found

this is My CollaboPDFController.php

<?php

namespace App\Http\Controllers;

use Illuminate\Http\Request;

use App\Http\Requests;
use App\Http\Controllers\Controller;
use PDF;

class CollaboPDFController extends Controller
{
    public function getPDF(){
        $pdf = PDF::loadView('customer.customer'); //line 14
        return $pdf->download('customer.customer');
    }
    //
}

how can fix this??

via Chebli Mohamed