First of all, if you look up the word focus in an English/English dictionary it says that in this case(refering to your sentence) focus can only be followed by on - not in. Otherwise it will not make any sense. As regards the in , it has to be in, because on the paper literally means that the types of objects lies on/upon the page.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.