It gets complicated, because protons and anti-protons are composite particles.

To simplify somewhat, one quark from each will annihilate, leaving four quarks and a lot of energy, which re-arrange into mesons (two quarks each), which then decay into photons, electrons, positrons and neutrinos.

Annihilation. At low collision energies the antiproton and proton go through an interaction which ultimately results in photons (light energy). Matter is converted into energy as e = mc^2. Though the interaction passes through a number of exotic intermediate stages first.

Someone has done the experiment: https://journals.aps.org/pra/abstract/10.1103/PhysRevA.108.0...

The link is cherry picked from a very good explanation by Gemini https://www.google.com/search?q=antiproton+hidrogen+colissio...