Few suggestions:
1.
if(!($baseDir . '\\AgcommandPortal\\agcommand\\php\\utils\\ISOxml\\' . $file)) {
Instead:
if(!file_exists($baseDir ....)){
2.Don't need size.
3.Try this one:
header('Content-Description: File Transfer');
header('Content-Type: application/octet-stream');
header('Content-Disposition: attachment; filename='.basename($fullpath));
header('Content-Transfer-Encoding: binary');
header('Expires: 0');
header('Cache-Control: must-revalidate, post-check=0, pre-check=0');
header('Pragma: public');
ob_clean();
flush();
readfile($fullpath);
exit;