All documents belong to the Government. Specifically the National Archives. So, it’s illegal for anyone to possess them. It doesn’t matter if they were a Congressman, Senator, Vice President or President that had proper security clearance. It doesn’t even matter if they were classified or not even though the right and Republicans tried to deflect saying he declassified them which is bullshit anyway as there are formal processes to declassify documents that ultimately end with the documents having a “declassified” stamp put on them which none of Trump’s documents had.. the point is, nobody can take documents, even a former President. The issues with the confidentiality and secrecy classifications is how much damage it could do or if it puts peoples lives at risk (e.g.CIA field agents).
The key difference between Biden and Trump was that once Biden’s legal team found the documents they immediately called the National Archives to come get them. They fully cooperated. It was the National Archives that then informed the DOJ given the political environment and what is going on with the Trump situation. In Trump’s case, he and his team stalled the National Archives, and it was pulling teeth to get Trump and his team to give over the documents. After asking if all Trump’s documents were returned and his legal team saying “yes” and a subpoena being issued it was found that they were not all returned and it appears Trumps staff tried to move move documents in and out of storage which then kicked off the FBI raid that found more documents. DOJ then appointed a Special Counsel to investigate to determine if Trump should be criminally charged with Obstruction.
There is a whole thread on the subject but I outlined the document timeline in this post
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The other issue is intent. What was Trump’s intent on keeping documents he had no legal right to have?