Hi Sergio,
I was reading an article the other day about browser support and one thing they mentioned was case within the header command.
It might be worth trying to follow the example laid out on the php.net website with forcing a download dialog on the user.
<?php
// We'll be outputting a PDF
header('Content-type: application/pdf');
// It will be called downloaded.pdf
header('Content-Disposition: attachment; filename="downloaded.pdf"');
// The PDF source is in original.pdf
readfile('original.pdf');
?>
As you can see they have combined the Content-Disposition into a single header call and also have capitals at the beginning of each line.
Like I said, not sure if this will solve it but might be worth a try.