Trump took classified documents after leaving office (military strategies and ongoing operations, nuclear weapon capabilities of the US and other countries, and other similar materials). He stored them at Mar-A-Lago without approval. On two specific occasions he showed the classified documents to staffers and journalists, even telling them “I’m not supposed to be showing you these”. Additionally, when the fbi told him to give the documents back he first denied having them, then gave them only some of the documents. When they asked for the rest, he only gave them some of the rest. I believe there are still some they haven’t found.