Save and download PDF

Why dont you swap them round create the pdf first as a file and then stream the created file back ?

$file_to_save = '/home/stsnew/public_html/pdf/file.pdf';
//save the pdf file on the server
file_put_contents($file_to_save, $dompdf->output()); 
//print the pdf file to the screen for saving
header('Content-type: application/pdf');
header('Content-Disposition: inline; filename="file.pdf"');
header('Content-Transfer-Encoding: binary');
header('Content-Length: ' . filesize($file_to_save));
header('Accept-Ranges: bytes');
readfile($file_to_save);

Answering late but might be will help onward.

on set attachment true or 1

$dompdf->stream("pdf_filename_".rand(10,1000).".pdf", array("Attachment" => true));

No need of file_put_contents() if you are just looking for download