public function actionCreate()
 {
    if (!Yii::$app->user->isGuest) {
        $model = new Product();

        if ($model->load(Yii::$app->request->post())) {
            $documentName=$model->id;
            $model->file=UploadedFile::getInstance($model,'file');
            $model->file->saveAs( 'uploads/'.$documentName.'.'.$model->file->extension);
            $model->document='uploads/'.$documentName.'.'.$model->file->extension;
            $model->save();

            return $this->redirect(['index']);
        } else {
            return $this->render('create', [
                'model' => $model,
            ]);
        }
    } else {
        $this->redirect('/site/index');
    }
}

<p><a href="<?= $product->document?>" download>Скачать файл</a>

public function actionCreate()
{
    if (!Yii::$app->user->isGuest) {
        $model = new Product();

        if ($model->load(Yii::$app->request->post())) {
            $model->file=UploadedFile::getInstance($model,'file');
            $model->file->saveAs( 'uploads/'.$model->file->baseName.'.'.$model->file->extension);
            $model->document='uploads/'.$model->file->baseName.'.'.$model->file->extension;
            $model->save();

            return $this->redirect(['index']);
        } else {
            return $this->render('create', [
                'model' => $model,
            ]);
        }
    } else {
        $this->redirect('/site/index');
    }
}