您可以使用属性filename:string(输出PDF文件名)更改文件名。
  
  $pdf = new Pdf([
        // set to use core fonts only
        'mode' => Pdf::MODE_UTF8, 
        //Name for the file
        'filename' => 'The_name_you_want',
        // A4 paper format
        'format' => Pdf::FORMAT_A4, 
        // portrait orientation
        'orientation' => Pdf::ORIENT_PORTRAIT, 
        // stream to browser inline
        'destination' => Pdf::DEST_BROWSER, 
        'marginTop' => 5,
        'marginLeft' => 5,
        // 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/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' => 'Customer Invoice'],
         // call mPDF methods on the fly
        'methods' => [ 
         //   'SetHeader'=>['Krajee Report Header'], 
           // 'SetFooter'=>['{PAGENO}'],
        ]
    ]);
    // return the pdf output as per the destination setting
    return $pdf->render();