How to send an message to the user after downloading

Hello,
I made a system that can make reports in Word or Excel format from data taken from a MySQL database.
The last command of my PHP script is:
$TBS->Show(OPENTBS_DOWNLOAD, $output_file_name);
Now I would like to send the user who just downloaded the file some information about the way to use this file.
I tried to add some javascript after this last command but to no avail.
I also tried to put a javascript alert just before the command $TBS->Show(OPENTBS_DOWNLOAD, $output_file_name);
But the result is: I get the alert correctly, but then the file is sent to the browser instead of being downloaded.

What would you suggest?
Thanks for your help.

HTTP do not let you download and display a HTML page.
A download is an HTTP query apart, without other client content before and after.

In other words: you can make actions at the server side before and after the HTTP download, but not at the client side.