While final opinions should only be based upon the entirety of circumstances, at initial blush, the FBI raid appears to be ill advised, if not illegal.  

The FBI's raid on Former President Trump's residence was allegedly spurred by "the Archivist" of the National Archives and Records Administration. It was, apparently, an attempt to gain possession of former President Trump's "15 boxes of documents".  In the absence of full knowledge of facts, the action presumably falls under the purview of the Presidential Records Act (PRA), 44 U.S. Code Chapter 22 - PRESIDENTIAL RECORDS.

However, PRA does not have criminal provisions for a former President's possession of allegedly archivable documents. PRA § 2206 provides, "The Archivist shall promulgate in accordance with section 553 of title 5, United States Code, regulations necessary to carry out the provisions of this chapter." But, neither § 2206 nor § 553 have criminal consequences for a 'former President.'

Before any criminality may attach, it must first be determined if the documents are "personal" as defined by § 2201 - Definitions, (3) and exempted from the act under § 2201 - Definitions (2)(b)(ii).
As  part of the publicized documents, the 'missives' from Kim Jong-un have been mentioned. These may, arguably, fall under the personal classification. Only two of these 27 documents have been released, publicly. The two released documents may be argued to be personal, in the nature of diary entries. Undoubtedly of historical significance. But not unquestionably official rather than personal.

Even if documents are determined by a court to be 'official' rather than 'personal', as an affirmative defense, former President Trump may, legitimately, claim access to the documents in controversy under PRA § 2205 - Exceptions to restricted access. Specifically, under  § 2205 "(3) the Presidential records of a former President shall be available to such former President or the former President’s designated representative." Such access does not preclude the former President from possessing records deemed or formerly deemed 'classified' at any level. The former President had the power and right to declassify any document, during his term of office. The president, as commander-in-chief, is ultimately responsible for classification and declassification. That does not apply to former presidents, but Mr. Trump was commander-in-chief when the documents were removed from the White House and can assert that he declassified all of the suspect materials before they were removed.  

The 1988 Supreme Court case 'Department of Navy vs. Egan' grants a sitting President the ability and authority to "classify and declassify at will." There is no specific protocol which must be followed. Therefore, no documentation needed to be created. Classification of official documents  and their declassification are derived from executive orders. But even executive orders aren’t obligatory on a president. The president is not obliged to follow any procedures for declassification, other than those that he has determined. And he can change those, at his discretion, without notice or advice of others. There’s no question that the president had broad authority to declassify almost anything at any time without any process, before leaving office. The former President needs now only assert that he declassified materials and documents in the 15 boxes before their removal. No one can prove that he didn’t.

 § 2205 (3) does  not restrict the time limits of availability nor the place of availability.

Under the PRA, former President Trump may not destroy documents but is entitled to the 'availability' of his Presidential documents during his natural lifetime. Upon former President Trump's death, the documents must be returned to, or released by, the Archivist.  

Oddly, there has been no outcry nor Federal raid on the material that Obama took when vacating the White House. That includes millions of dollars in jewels from the Saudi royal family, a surfboard from the prime minister of Australia, and several ceremonial weapons from various world leaders. And enough documents to fill a 74,000 square foot warehouse in Chicago. Documents reportedly equal to 200 terabytes of electronic records. While all materials are technically owned by the National Archives, Obama retains possession. The material was supposedly intended for an Obama Presidential Library which is still not in operation, more than five years after Obama left office. Similarly, massive amounts of materials were removed by former presidents Reagan, G H W Bush, Clinton, and G W Bush. All much greater than the ’15 boxes’ which supposedly warranted an unprecedented search of a former president’s home.

As opposed to former Clinton Administration National Security Advisor "Sandy" Berger who pleaded guilty to one count of violating 18 U.S.C. § 1924, "Unauthorized removal and retention of classified documents or material." No criminality can be attached to Trump, any more than to Regan, Bush 41, Clinton, Bush 43, or Obama. That’s because § 1924(a) conditions the offense upon “without authority” to access the documents. As above, the PRA grants authority to former presidents to access the presidential documents of his administration.

The only malicious act by the former President would be the destruction of documents. If they had been destroyed, there would be no reason for a search warrant to seek documents no longer in existence.

Part of his controversy will pivot on the location of the documents, in an attempt to attach criminality.
Apologies are being given for Obama's retention of documents because it was 'authorized' by the National Archives,
 
While many have commented upon the previous ongoing dialogue between Trump's representatives and the National Archives, few have opined on the implications.
 
1) Mar-a-Lago is Federally protected by the Secret Service 24/7 x 365 by former President Trump's security detail, hence a federally secured facility.
2) When the National Archive staff instructed Trump representatives to put additional padlock(s) on the door to the room where the documents were being held, it was an explicit instruction; constituted a 'substantive rule'; and an implied approval for the documents to remain at Mar-a-Lago. (See, e.g., US v Dougherty)
3) The National Archives agent, by giving instructions for the padlock, was exercising operational control over the storage location.
4) Therefore Trump, through his representatives, was in compliance with National archive instructions and was storing documents with the approval of the National Archives.  

This should militate criminality for documents being located at Mar-a-Lago.

The only conclusion should be that Trump was authorized to access the records, by statutory law. Security clearances do not apply to a former president who has declassified the documents. And possession of the documents at Mar-a-Lago was under the operational control of the National Archives and Federal Secret Service protection.
 

So, just what was the reason for a search warrant?