I find it fascinating to see the pages he wrote 350 years ago, before he was famous. Most impressive to me is that if you look at page 5 of his Trinity College Notebook, you see what looks to me like a doodle! Newton doodles? I hadn't really thought much about what he must have been like as a young man in college. Did his mind wander? Did he think visually? To me, his handwriting looks organized, but not overly neat or orderly. It is clear that before computers and calculators, mathematics involved a lot of calculations. For some reason, I'm particularly pleased to see the places where he scribbles things out and makes corrections.
See for yourself here.