See, here is the thing: We, as the public, will never know what these documents are about. They can't tell us because they are secret. Trump could have had information on spies active in Russia. He could have detailed information on next generation weapon systems. He might have assessments on allies and their abilities. Or, as I previously said, it could be the lunch menu from 2018. We will never know in detail what they are.
All that being said, if Trump possessed sensitive materials and refused to return them, even after a subpoena, they why shouldn't the FBI search the premises for these sensitive documents? I mean, what would you have said if Hillary retained top secret or classified data? You would chant "LOCK HER UP". But...strangely, you don't feel the same way when it comes to Trump. It is so weird how that works out....