First one: “I’m two pages in and I still have no idea what you are saying”
Second one: “I’m in two pages and I still have no idea what you are saying”
The first is idiomatic, and what most native speakers would say.
The second can be misconstrued as “I’m inside two pages”, and although that doesn’t make sense, it means you have to re-read in order to get the correct interpretation.
Source : Link , Question Author : Alex Vasilev , Answer Author : MonkeyPushButton