$mpdf->Output($filename, MpdfOutputDestination::FILE);
//send the file inline to the browser.
MpdfOutputDestination::INLINE, or "I"
//send to the browser and force a file download.
MpdfOutputDestination::DOWNLOAD, or "D"
//save to a local file
MpdfOutputDestination::FILE, or "F"
//return the document as a string
MpdfOutputDestination::STRING_RETURN, or "S"