It may help to be a little more descriptive. For instance, downloaded only once a day per person, or in total? Also, do you have access to MySQL? My suggestion would be to write the script yourself, so you know that it works exactly as you want it.
Since this board is mainly for people who are wanting to learn/need help with PHP, perhaps it would be better to ask how to write the script(s).
I would suggest using a MySQL database to store the date of the last access time, then compare the two (the old and the one being visited). If there is more than a 24 hour difference, then allow the file to be sent and then reset the file access time.