Kartik mpdf report blank

I have this form

    <?= Html::beginForm(
        Url::toRoute("caja/imprimir"),
        "get",
        ['class'=>'form-inline']
        );
    ?>

        <div class="form-group">
        <table class="table table-bordered table-hover">
                <thead>
        <tr>
			<td><strong>Fecha Inicial</strong></td>
			<td>&nbsp;<?= DatePicker::widget(['name' => 'fechainicio', 'dateFormat' => 'yyyy-MM-dd']) ?></td>
		</tr>
		<tr>
			<td>&nbsp;<strong>Fecha Final</strong></td>
			<td>&nbsp;<?= DatePicker::widget(['name' => 'fechafin', 'dateFormat' => 'yyyy-MM-dd']) ?></td>
		</tr>
		<tr>
			<td>&nbsp;<strong>Ruta</strong></td>
			<td>&nbsp;<?= Html::dropDownList('id', null, 
     		ArrayHelper::map(Rutas::find()->all(), 'id', 'nombre'), ['class' => 'form-control', 'prompt'=>'Selecionar...']) ?></td>
		</tr>
        </div>

        </table>
        <?= Html::submitInput("Generar", ['class' => 'btn btn-danger'])?>

       <? Html::endForm()?>

Controller

    $desde = date("Y-m-d", strtotime($_GET['fechainicio']));
     $hasta = date("Y-m-d", strtotime($_GET["fechafin"]));
     $ruta = Yii::$app->request->get("ruta");
     $Ventas = Ventas::find()->andwhere(['>=','fecha',$desde])->andwhere(['<=','fecha' ,$hasta])->andwhere(['=','idruta',$ruta])->all();
     $content = $this->renderPartial('_report', ['Ventas' => $Ventas]);

     $pdf = new Pdf(['format' => Pdf::FORMAT_A4, 'orientation' => Pdf::ORIENT_PORTRAIT, 'destination' => Pdf::DEST_BROWSER, 'content' => $content, 
        //'cssFile' => '@vendor/kartik-v/yii2-mpdf/assets/kv-mpdf-bootstrap.min.css', 
        'cssInline' => '.kv-heading-1{font-size:18px}', 
        'options' => ['title' => 'Krajee Report Title'], 
        'methods' => [ 
        'SetFooter' => ['{PAGENO}']]]);
     // return the pdf output as per the destination setting
     return $pdf->render();

and print the pdf but it comes out blank.
I appreciate your helpmeā€¦