Thanks Anna for your quick reply.
I have just tested that PHP is not limited in download, so I will looking for a solution to let my user download a file saved in splitted parts...
... now I am serving the download file streaming them via the following PHP commands:
PHP Code:
header('Content-Description: File Transfer');
header('Content-Type: application/octet-stream');
header('Content-Disposition: attachment; filename="' . basename($file) . '"');
header('Content-Transfer-Encoding: binary');
header('Expires: 0');
header('Cache-Control: must-revalidate, post-check=0, pre-check=0');
header('Pragma: public');
header('Content-Length: ' . filesize($file));
ob_clean();
flush();
readfile("$file");
but I will find a way to concatenate multiple files in a single stream 
Of course... please take my user in mind if you will let some user to have unlimited file size again, thanks.
Lorenzo
(I think you won't decide to elevate my account from "prime" to "premium" at no cost... but, as you say, it will be a good solution for me)