with him after leaving office...he should have turned them over...they don't belong to him. This attempt to get those documents has been ongoing since April...an invasive search like this...especially of a former President...takes a fair amount of time to justify to a federal judge, so I'm guessing the FBI has been building its case for some time and is quite concerned.
Not sure if we'll know what documents the FBI is most concerned about, but Trump is not known for being respectful of the law over his own interests, which makes me wonder if he was using those docs for his own benefit, as opposed to the nation's.