pjax prevents using $app->response->sendFile in Controller

Following code

$completePath = Yii::getAlias('@pictures' . '/' . $filename);return Yii::$app->response->sendFile($completePath, $filename);

will do its job if I code GridView without pjax. If I use pjax like this:

 <?=

    GridView::widget([

        'dataProvider' => $dataProvider_vermieten,

        'filterModel' => $searchModel,

        'columns' => $gridColumn_vermieten,

        'pjax' => true, //this code prevents rendering pdf

        'pjaxSettings' => ['options' => ['id' => 'kv-pjax-container-immobilien']],

        'panel' => [

            'type' => GridView::TYPE_PRIMARY,

            'before' => Html::a('<i class="glyphicon glyphicon-plus"></i> zur Hauptseite', ['/site/index'], ['class' => 'btn btn-info']),

            'after' => Html::a('<i class="glyphicon glyphicon-repeat"></i> Reset Grid', ['/immobilien/index'], ['class' => 'btn btn-primary', 'title' => 'Setzt die GridView zurück']),

            'heading' => '<span class="glyphicon glyphicon-book"></span>  ' . Html::encode($this->title),

        ],

        // your toolbar can include the additional full export menu

        'toolbar' => [

            ['content' =>

                Html::a('<i class="fa fa-folder-open"></i>', ['/immobilien/index', 'bez' => 'umbenennen'], ['class' => 'btn btn-primary', 'title' => 'additional content'])

            ],

            '{export}',

            '{toggleData}'

        ],

        'toggleDataOptions' => ['minCount' => 10],

    ]);

    ?>

pdf file won’t be rendered. Any ideas, how to render pdf-file using pjax in spite of everything?