What a MIDI made from a PDF is good for
A MIDI file is a performance, not a page: which key goes down when, and for how long. Made from a PDF, it is the score played exactly as written, which is what you want for practising along at a slower tempo, checking a passage you cannot hear in your head, or starting an arrangement in a DAW.
Each staff becomes its own track, so in a piano piece you can mute the left hand and play it yourself. The printed tempo mark is used when there is one.
It reads PDFs exported from notation software (MuseScore, Sibelius, Finale, Dorico, LilyPond, Guitar Pro), where every note is a symbol in the file. A scan or a photo of paper is a picture, and it says so instead of guessing.
If you will open the result in notation software rather than a DAW, choose MusicXML instead: it keeps the key, the spelling of every accidental and the layout, which MIDI cannot carry.