'Content-Disposition: attachment; filename="' . $fileNameDownload .
'.' . $this->_extension . '"'
has been fixed in the latest version.
Please note that PHPDocX has createDocxAndDownload method to allow the download of the document. But of course, the headers that this method uses depend on your server. We added createDocxAndDownload but if this method doesn't work in your server you can use createDocx and the add a few lines to download the document.
For example if you integrate PHPDocX in Drupal we don't recommend you to use createDocxAndDownload but the included functions in this CMS.