First, we do not know yet what the classified documents found in Biden's office at the Penn Biden Center contain, their level of classification, or how many they are, while we know how many were found in Trump's possession.
Second, Biden and Trump were covering different roles at the time of their handling of the classified documents. Trump's classified documents date back to the time of his presidency, while the documents found in Biden's office belong to the time of Biden's vice presidency under then-President Obama.
Third, Trump as president had the power to declassify the documents—something that Biden as vice president at the time could not do. There's no evidence that Trump issued an official order to declassify the documents found at Mar-a-Lago last year.
A fourth, crucial difference between the two cases is the way Trump's and Biden's classified documents were obtained by authorities.
Biden's lawyers notified NARA on the day they found the documents at the Washington DC think tank, while affidavits linked to last August's FBI raid of Mar-a-Lago show that Trump's team did not fully cooperate with NARA in handing over the documents held at the Florida resort.
Documents show that officials tried to retrieve the sensitive material taken by Trump after leaving office for months without success, leading the Justice Department to escalate the matter to the FBI.
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https://thehill.com/homenews/administration/3806250-differences-in-the-trump-biden-classified-document-discoveries/