That's easy. If the Democrats want to compel the president to "cooperate" they have Congressional powers to do so - by actually voting to impeach him. They can also petition the Supreme Court to force the President to release information. Those are their "means of compulsion". At the moment, the Democrats are abusing the investigatory process by making demands of the president while denying him the proper rights that a defendant should be afforded. Considering how contrived the complaint upon which the inquiry is predicated is, that is hardly surprising.
See, the problem the Democrats have - which Pelosi knew from the beginning - is that any impeachment trial is just as likely to damage the them as it is to damage Trump. So instead she's trying to use the investigatory phase as a way of smearing the president whilst not actually having to commit to the process.