I'm pretty sure we went over this last year. There is no law which describes limits to, or even a procedure for, the declassification powers of a president. If a president wants to drop off classified papers at a little old widow's house in Kansas, who does not have security clearance, and imply that this is the appropriate spot for them, he has the power to do that. In the past presidents have given people top secret clearance on the spot by motioning them into classified briefings.
Trump was president when he moved the papers to Mar-a-Lago, therefore this is the appropriate spot for those papers. Notice that the document Markjo linked does not actually cite any laws on what a president can and can't do with classified materials. It has been said that there are no laws because the chief executive is the body from which the very nature of classification originates. The executive is classifying the material, and so the executive has all power over the nature of classification.
The document tries to avoid admitting that a president with unlimited declassification power gave those papers to himself, deciding that he should have them even after he knew he would no longer be president.