class WebservicesTestBlock extends BlockBase {

    public function build() {

        $form = Drupal::formBuilder()->getForm('Drupalwebservices_testFormWebservicesTestForm');
        return $form;

    }

}

public function submitForm(array &$form, FormStateInterface $form_state) {


    // here is the code that returns pdf dats

        header("Content-Type: application/pdf");
        header("Content-Disposition: attachment; filename=test.pdf");
        header('Content-Transfer-Encoding: binary');
        //header("Cache-Control", "max-age=0");
        header('Accept-Ranges: bytes');
        ob_clean();
        flush();
        echo ($test->body);
        exit;

    }