The argument is that it's up to the outgoing President to decide what is Presidential and what is Personal, and Trump (and Trump-lovers) have trouble understanding the distinction. In some cases, I can see that it's a possible grey area. But, classified documents? Really?
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The term "personal records" means all documentary materials, or any reasonably segregable portion thereof, of a purely private or nonpublic character which do not relate to or have an effect upon the carrying out of the constitutional, statutory, or other official or ceremonial duties of the President. Such term includes--
(A) diaries, journals, or other personal notes serving as the functional equivalent of a diary or journal which are not prepared or utilized for, or circulated or communicated in the course of, transacting Government business;
(B) materials relating to private political associations, and having no relation to or direct effect upon the carrying out of constitutional, statutory, or other official or ceremonial duties of the President; and
(C) materials relating exclusively to the President’s own election to the office of the Presidency; and materials directly relating to the election of a particular individual or individuals to Federal, State, or local office, which have no relation to or direct effect upon the carrying out of constitutional, statutory, or other official or ceremonial duties of the President.
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The documents can in no way be understood to be "personal records". The fact that a President of the United States of America believes them to be, and so many supporters blindly (i.e. ignoring facts) follow every word is either very, very concerning for humanity or just plain disingenuous.
The above case was about NARA's refusal to designate the Clinton tapes as 'Presidential' after the fact - and it was decided they have no power to do so. Therefore it makes sense that NARA can't designate what Trump took as 'Presidential' after the fact.
But anyway, Trump's hasn't been indicted over the Presidential Records Act.
The fact is that Trump is indicted under the Espionage Act.