It is clearly illegal, and the case has been referred to the Department of Justice by the National Archivist. Keep in mind that General Petraeus was convicted for merely bringing his notes home, not actual docs.
But my point here isn't really about Trump. What about those who shouted "Lock her up" when Hillary legally stored emails on a private server that was never ever hacked? What do they have to say now?
My take is those who excuse Trump and vilify Hillary should never vote again, should get sterilized, and if that is too late, they should urge sterilization for every younger person who shares their DNA. That would improve things.