Typo3 PDF only viewable if Logged into Backend

Question

I run into a Problem with PDF creation on Typo3.

I have a Website with a lot of different Tables. Some of them should be available as PDF Exports.

The best way (for me) was to use a Custom ViewHelper with a special Controller and everything.

This works IF Iam loggend in into the Backend. It will not Work if Iam loggen in as a FE User. In this case the PDF is opened with a warning that it could not be displayed...

Is there a Setting I need to change?

Edit:

This is the ViewHelper

class PDFViewHelper extends AbstractViewHelper {

public function initializeArguments(){
    [...]
}

public static function renderStatic(
    array $arguments,
    \Closure $renderChildrenClosure,
    RenderingContextInterface $renderingContext
) {
[...]
}

private static function generateReport(){
{
    $file = new FPDF();
    [...]
    return $file->Output();
}

}

The Template

{namespace exsv=basicNamespace\ViewHelpers}

<exsv:PDF [...]/>

And the ActionController

class PDFAPIController extends ActionController{

protected $viewFormatToObjectNameMap = array(
    'pdf' => 'basicNamespace\Classes\ViewHelper\PDFView'
);

public function createPDFAction(){
    /** @var \TYPO3\CMS\Frontend\Controller\TypoScriptFrontendController $typoScriptFrontendController */
    $typoScriptFrontendController = $GLOBALS['TSFE'];
    $typoScriptFrontendController->setContentType('application/pdf');
    $this->response->setHeader('Content-Transfer-Encoding', 'binary');
    $this->response->setHeader('Content-type', 'application/pdf');
[...]

$this->view->setControllerContext($this->getControllerContext());
}

}

Edit2:

Some further investigation directed me to the followeing line.

$typoScriptFrontendController->setContentType('application/pdf');

everything is fine until I comment this out in the controller.