Yii2: renderPartial error => error on line 1 at column 1: Document is empty - layout

Question

I am using Krajee mdpf tool(http://demos.krajee.com/mpdf) on Yii 2. I tried to generate a view and I succeed with the render() function. The problem is that it displays a layout. So I decided to use renderPartial() function to only display the htlm of the view but I get this error:
This page contains the following errors:
error on line 1 at column 1: Document is empty
error on line 1 at column 1: Encoding error
Below is a rendering of the page up to the first error.
Here is my actionPdfTest function:
public function actionPdfTest()
{
// get your HTML raw content without any layouts or scripts
$content = Yii::$app->controller->renderPartial('_example');
// setup kartik\mpdf\Pdf component
$pdf = new Pdf([
// set to use core fonts only
'mode' => Pdf::MODE_CORE,
// A4 paper format
'format' => Pdf::FORMAT_A4,
// portrait orientation
'orientation' => Pdf::ORIENT_PORTRAIT,
// stream to browser inline
'destination' => Pdf::DEST_BROWSER,
// your html content input
'content' => $content,
// format content from your own css file if needed or use the
// enhanced bootstrap css built by Krajee for mPDF formatting
'cssFile' => '#vendor/kartik-v/yii2-mpdf/src/assets/kv-mpdf-bootstrap.min.css',
// any css to be embedded if required
'cssInline' => '.kv-heading-1{font-size:18px}',
// set mPDF properties on the fly
'options' => ['title' => 'Krajee Report Title'],
// call mPDF methods on the fly
'methods' => [
'SetHeader'=>['Krajee Report Header'],
'SetFooter'=>['Krajee Report Footer'],
]
]);
// return the pdf output as per the destination setting
return $pdf->render();
}
Code of _example view:
<?php
use yii\helpers\Html;
use yii\widgets\ActiveForm;
/* #var $this yii\web\View */
/* #var $form yii\widgets\ActiveForm */
/* #var $model app\models\LoginForm */
$this->title = 'Hello World';
?>
<h1><?= Html::encode($this->title) ?></h1>
Please Can you help me?

Answer 1

Thank you every body,
I found the solution. It was to add :
Yii::$app->response->format = \yii\web\Response::FORMAT_RAW;
before the:
$content = Yii::$app->controller->renderPartial('_example');
Here is the final code:
public function actionPdfTest()
{
Yii::$app->response->format = \yii\web\Response::FORMAT_RAW;
// get your HTML raw content without any layouts or scripts
$content = Yii::$app->controller->renderPartial('_example');
// setup kartik\mpdf\Pdf component
$pdf = new Pdf([
// set to use core fonts only
'mode' => Pdf::MODE_CORE,
// A4 paper format
'format' => Pdf::FORMAT_A4,
// portrait orientation
'orientation' => Pdf::ORIENT_PORTRAIT,
// stream to browser inline
'destination' => Pdf::DEST_BROWSER,
// your html content input
'content' => $content,
// format content from your own css file if needed or use the
// enhanced bootstrap css built by Krajee for mPDF formatting
'cssFile' => '#vendor/kartik-v/yii2-mpdf/src/assets/kv-mpdf-bootstrap.min.css',
// any css to be embedded if required
'cssInline' => '.kv-heading-1{font-size:18px}',
// set mPDF properties on the fly
'options' => ['title' => 'Krajee Report Title'],
// call mPDF methods on the fly
/*
'methods' => [
'SetHeader'=>['Krajee Report Header'],
'SetFooter'=>['Krajee Report Footer'],
]
*/
]);
// return the pdf output as per the destination setting
return $pdf->render();
}

Answer 2

this error Message mostly appears, if there is no correct handling of http-header, document-format and/or sending output to the browser.
I don't know, what return $pdf->render(); exactly is doing. But I guess your problem is a mix of sending wrong header, sending header multiple times and/or sending output that doesn't matches the header.
So you should think about:
What output-type you expect for your script? Check, if you set the http-Header that matched your output type.
Check if $pdf->render(); outputs something more than the return value.
If $pdf->render() returns an pdf, you should send this as file to the browser, like that ("pseudo-code"). Using sendFile, sendContentAsFile or sendStreamAsFile depends on the output of $pdf->render() ( https://www.yiiframework.com/doc/guide/2.0/en/runtime-responses#sending-files )
return Yii::$app->response->sendContentAsFile(
$pdf->render(),
"download.pdf",
['inline' => true,
'mimeType' => 'application/pdf']