Strictly speculation here. But, what if the motive not to return the documents for so long was because of empty folders? If any of those documents were traded for favors or money, he would need to hide the evidence of the crime that carries a death sentence. Not wanting to leave behind evidence, he took as many boxes as he could to search for them later. It’s possible that he could not find all of the empty folders until the FBI did. When he claimed that mostly all he took was empty folders, it may have been an unconscious projection or gaslighting revealing his motives. The article is a bit dated, but those empty folders have been mentioned recently in discussions and news about the Mar a Lago docs case, and this article shines a little extra light on that particular detail. Oddly, there is even a claim that one of these folders was used as a lampshade of sorts, in his bedroom. Why would he have one of these in his bedroom? My guess is consternation and fear.