No he actually didn't declasify them. The president has the authority to order documents to be Declassified, But they are not officially declassified until their markings are altered. The fact that these documents still had classification markings means that they were still officially classified.Trump declassified those documents that you are saying were classified. The corrupt FBI still went after him.
The truth is, Trump will never be charged because he had the authority to take those documents.
And stealing presidential documents is a felony re gardless a whether they're classified or not. Under federal law specifically the Presidential records act, All Official documents belonging to the White House have to be handed over to the national archives when a president leaves office. It is a felony to illegally retain official documents without permission regardless of whether they are classified or not.