Trump, Biden, and now Mike Pence have all been caught with unsecured classified documents. This illustrates that it is not unusual to mishandle classified documents. It often happens with people who handle them frequently.
In American embassies there are Marine guards who patrol embassy offices every night looking for classified documents and they often find some. If you are not careful it is easy to get classifed documents mixed up with unclassified papers. It sounds like Trump deliberately kept some documents, but it appers that Biden and Pence probably just got some classified documents mixed up with some unclassified ones. They may have been working on an issue and taken some papers home to continue to work on it, and unknowingly mixed some classified documents with the papers they took home.
Is is wrong and should be punished, but in most cases it is not a crime. The State Department issues security violations for mishandling classified information, but usually there is only some administrative punishment, and then only after several security violations.
Trump seems to be guilty of deliberately retaining classified material in an unsafe manner, but there is so far no indication that he disclosed it to a foreign government, or used it in a way that damaged the security of the United States. While Trump’s actions seem worse, Biden’s and Pence’s make it difficult to punish Trump for what he did, unless something comes out that he seriously damaged national security. He should get a slap on the hand, but no criminal charge.