This right there is the difficult part - what do you mean exactly? I cannot come up with anything better than search, as in like Google search. And they did it for books already, it's seriously good.
A big problem that happens with FOIA requests is that you're often sent data in the form of a spreadsheet that was converted to a PDF. And then scanned. For thousands of pages. Solve that generally so that you can insert all of that data into a postgres database, with sensible indexes.
This right there is the difficult part - what do you mean exactly? I cannot come up with anything better than search, as in like Google search. And they did it for books already, it's seriously good.