just set the cookie experation time
Cookie calculations are
seconds X minutes X Hours X Days
I want to have a cookie to expire after 20 minutes, so the following will be:
60 X 20
setCookie('CookieValue', $value, time()+(1200), '/', '', 0);
so your page will have
if (!$HTTP_COOKIE_VARS['CookieValue']) {
echo "<a href=\"Your link to file\">Download Now</a>";
} else {
echo "You have already downloaded this file";
}