They find classified files in boxes that Trump had taken from the White House

This was explained by the head of NARA, David Ferriero, in a letter of response to several questions asked by Congresswoman Carolyn Maloney after it was learned that in January these official documents were recovered at the residence of the former president in Florida.
“The National Archives Administration has identified classified national security information in the boxes,” Ferriero confirmed in his letter, explaining that the Justice Department has been informed of this finding.
National Archives continues to analyze the contents of the 15 boxes, which he recovered last January from Trump’s residence in Mar-a-Lago (Florida) and expects to conclude the process on February 25.
Some documents could not be reconstructed
Ferreiro also confirmed, as several media had advanced, that Trump used to break official documents into pieces, despite the fact that the presidents are obliged to deliver them to the National Archives when they leave the White House.
“Although white house staff during the Trump administration pasted some documents, others they sent us were not reconstructed,” the official said.
When they leave the White House, U.S. presidents must deliver to the National Archives all letters, notes, emails, and other written communications that relate to their official duties as president for preservation.
Likely violation of the Presidential Records Act
The fact that Trump took those boxes to Florida is a likely violation of the Presidential Records Act, which requires the preservation of such documents.
However, federal prosecutors would have to prove that he intentionally manipulated confidential information or committed extreme negligence with it, something complicated, and more so in the case of a president who can declassify data at will.
Among the documents the former president took away was correspondence sent to him by North Korean leader Kim Jong-un during the bilateral thaw process, which Trump once described as “love letters.”
After being uncovered last week in several media, Trump said in a statement that the documents he had taken “were delivered easily and without conflict and in a very friendly way” to the National Archives and that this material will one day be exhibited in the presidential library that will bear his name.

Original source in Spanish