Frederic Henry is the narrator. Unless he describes another character’s reaction to something, every thing we see is filtered through his eyes. He’s not a villain and the story is his personal confession, so he’s a likely candidate for protagonist.
Some critics think Catherine Barkley steals the show from Frederic and is the real hero of the novel. Since Frederic and Catherine keep talking about being the same person and sharing everything, we can go ahead and let them share the limelight. Frederic probably would have wanted it that way. Actually, he probably would have nominated Catherine as the protagonist, since he gets bashful over awards and stuff.