Tip - Look for the larger [sized] files. They typically will have more complex arrangements.
If you are using these files to make money, be sure to contact the file creator and copyright owner for proper licensing.
There is usually information about who created the file embedded in the track names. If you use a file professionally (make money from it), you should try to contact the creator to see if you need to license the MIDI sequence/file. If the sequence is good enough for you to want to use, then the creator should be rewarded for their amazing work (just maybe they'll do more). Often they will let you use it free of charge but don't make that assumption unless you've made contact with the creator of the sequence.
You may have noticed that we used the term "file creator" instead of author or owner. This is because the author/owner of a song is often someone completely different than the creator or a MIDI sequence (arrangment). Sometimes the author is the owner of the song but not always. Authors can be hired to create music on someone elses behalf and that someone else is the copyright owner of the resulting music.
Like the creator of the MIDI sequence, the copyright owner of the song expects to be paid for their property (you wouldn't expect to go to work and not get paid) so, for professional use of a song, you also need to contact the owner of the song's copyright.
If no money is being created by your use of a song/sequence (just using it for fun), then you usually don't need any licensing. This doesn't mean however, that you can use a song for free just because you are not being paid. If you are playing at a bar for free, the bar is still making money so the songs/sequences need to be licensed.
It may seem like a lot of extra work when, in a lot of cases, you are not even making any money but it does encourage you to create your own music and will also put you in contact with the copyright owners which can have side benefits you hadn't considered (business contacts, etc.). Also helps you understand the business better.