I do find Tom's actions to be believable. I think that it is relatively easy for us to talk ourselves into doing things that we probably should not do. That is what Tom does in the story.
At first, when the paper flies out the window and gets stuck in the corner, he's just going to give it up. But little by little, he starts to notice things that would make it plausible for him to go out and get it. He starts to think that with the width of the ledge and the projections in the wall for handholds, it's not that big a deal. As he says to himself,
...if this ledge and wall were only a yard above ground ... he could move along the ledge indefinitely.
So I think this is fairly plausible. Tom is a young and fit man. He doesn't want to waste all the work he has put into the paper. I can see myself in that sort of a situation convincing myself that it makes sense to go out on the ledge and I think it would be even easier to convince myself of that if I had Tom's more hard-charging personality.