It shouldn't be hard to find the file if it's in the proper room! The files are all alphabetically sorted by name and there's labels on the cabinets (think "Libra Users A-G") and so on.

As per what the post says, if they are just looking at the files they got 30 minutes before the Reaper shows up. If they attempt to steal or destroy the files, the reaction is pretty instanteously and they got maybe a minute before velociraptors start rushing in.

Hope this helps!