在Yii框架2的kartik/mpdf中，当文本过长时，字体大小会变小.如何解决这个问题

使用Yii框架2，我使用kartik'mpdf'Pdf类将PDF格式的HTML页面响应到浏览器。下面是我的配置。

$pdf = new kartik'mpdf'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' => $htmlContent,  
                // 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/assets/kv-mpdf-bootstrap.min.css',
                // any css to be embedded if required
                'cssInline' => '.kv-heading-1{font-size:18px} .page-break{ page-break-after:always; } li { list-style:none; }', 
                 // set mPDF properties on the fly
                'options' => ['title' => 'My title'],
                 // call mPDF methods on the fly
                'methods' => [ 
                    'SetHeader'=>['My_header_ ' . date('d-m-Y')], 
                    'SetFooter'=>['{PAGENO}'],
                ],
 ]);
 // return the pdf output as per the destination setting
 return $pdf->render(); 

一切工作完美，除了当有一个变量在我的HTML页面，它返回一个长字符串，文本变小。我怎么解它?