One way to do this is to force the browser to display the "download file"-dialog. So the user can decide what to do with the file.

This can be done via PHP-Headers. (http://www.php.net/manual/en/function.header.php#83384)

There is also an example on how to this (Post 83384):

<?php
    // downloading a file
    $filename = $_GET[ path ];

    // fix for IE catching or PHP bug issue
    header("Pragma: public");
    header("Expires: 0"); // set expiration time
    header("Cache-Control: must-revalidate, post-check=0, pre-check=0");
    // browser must download file from server instead of cache

    // force download dialog
    header("Content-Type: application/force-download");
    header("Content-Type: application/octet-stream");
    header("Content-Type: application/download");

    // use the Content-Disposition header to supply a recommended filename and
    // force the browser to display the save dialog.
    header("Content-Disposition: attachment; filename=".basename($filename).";");

    /*
    The Content-transfer-encoding header should be binary, since the file will be read
    directly from the disk and the raw bytes passed to the downloading computer.
    The Content-length header is useful to set for downloads. The browser will be able     to
    show a progress meter as a file downloads. The content-lenght can be determines by
    filesize function returns the size of a file.
    */
    header("Content-Transfer-Encoding: binary");
    header("Content-Length: ".filesize($filename));

    @readfile($filename);
    exit(0);
?>

When using this example please consider that using

$filename = $_GET[ path ];

is a big security problem. You should work with something like ID s instead or validate the input. For example:

if($_GET[ file ] == 1) {
    $filename = foobar.pdf;
} elseif($_GET[ file ] == 2) {
   $filename = foo.pdf;
} else {
   die();
}