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<?php

public function actionRenderBoleto()
{
    \Yii::$app->response->format = \yii\web\Response::FORMAT_RAW;
    \Yii::$app->response->headers->add('content-type','application/pdf');
    \Yii::$app->response->data = file_get_contents('file.pdf');
    return \Yii::$app->response;
}