change the dompdf download folder

The question contains very little about your problem ideally you give more details, but let's try to help, are you trying to set the folder for the client-side? If yes, you can not tell the client where it will force the download by stream, it will always save where the user says to save, you can only set the location on your server. If I'm on your server you can tell it like this:

  $dompdf->stream("pasta/doc/documento_para_download.pdf", array("Attachment" =>true));