"require": {
  "php": ">=5.4.0",
  "yiisoft/yii2": "*",
  "yiisoft/yii2-bootstrap": "*",
  "yiisoft/yii2-swiftmailer": "*",
  "mpdf/mpdf":"*"
},

composer update

php composer.phar require mpdf/mpdf "dev-master"

'mPDF' => array($vendorDir . '/mpdf/mpdf'),

//引用自定义类方法
Yii::$classMap['mPDF'] = '@common/class/mpdf/mpdf.php';

...............
use app\models\ContactForm;
use mPDF;
class SiteController extends Controller
{
...............
  public function actionCreateMPDF(){
    $mpdf=new mPDF();
    $mpdf->WriteHTML($this->renderPartial('mpdf'));
    $mpdf->Output();
    exit;
    //return $this->renderPartial('mpdf');
  }
  public function actionSamplePdf() {
    $mpdf = new mPDF;
    $mpdf->WriteHTML('Sample Text');
    $mpdf->Output();
    exit;
  }
  public function actionForceDownloadPdf(){
    $mpdf=new mPDF();
    $mpdf->WriteHTML($this->renderPartial('mpdf'));
    $mpdf->Output('MyPDF.pdf', 'D');
    exit;
  }
...............
}