Problem saving pdf with wkhmltopdf - Cakephp 3

I'm using CakePdf

The documentation gives this example to create inside a folder:

<?php
    $CakePdf = new \CakePdf\Pdf\CakePdf();
    $CakePdf->template('newsletter', 'default');
    $CakePdf->viewVars($this->viewVars);
    $CakePdf->write(APP . 'files' . DS . 'newsletter.pdf');

I replaced by:

$CakePdf->write("/webroot/img/" . 'newsletter.pdf');

However, the file is not created. I am grateful if someone can analyze me or indicate a way to implement this!