Create a flow that is based on a schedule. Each night, connect to the library, loop through all items and check to see if the modified date meets your criteria, and send the email. Of course, this will send another email the next night again if no one modifies the file. Or, better, configure the "get items" action to use a filter so that it only gets the items that meet your condition.
Instructions for setting up a flow to run on a schedule are here. Instructions for getting items from a list that are filtered by date, instead of all items, are here.