First - does your code produce a working XML based docx file that you can open with word? If that doesn't work, then don't worry about your header information. Once you've successfully created a docx file, then you can use the header info. Suggestion: use one of the example files included w/PHPWord and, if you get that to work, then add the header block.
The following snippet works for me (at least as of last week):
$objWriter = PHPWord_IOFactory::createWriter($PHPWord, 'Word2007');
$filename = "Perfomance_Appraisal.docx";
$objWriter->save($filename);
header('Content-Description: File Transfer');
header('Content-Type: application/octet-stream');
header('Content-Disposition: attachment; filename='.$filename);
header('Content-Transfer-Encoding: binary');
header('Expires: 0');
header('Cache-Control: must-revalidate, post-check=0, pre-check=0');
header('Pragma: public');
header('Content-Length: ' . filesize($filename));
flush();
readfile($filename);
unlink($filename);