Each of these answers a question we kept seeing asked and not answered well. They are written to be useful on their own, including in the cases where the answer is a tool we do not make. Updated in place rather than replaced by a newer post.
Four routes that actually work: the search already in your tablet, converting to text, photographing paper, and indexing the strokes. What each costs, and where each breaks. Three of the four are not us. Plus the three things that break every route, which most guides skip.
Privacy here is argued and almost never measured. Four states to put a device in, what the traffic means, and how to settle it for your own tablet in an evening. Deliberately a method rather than a league table: we make software in this category, so our ranking would be worth nothing.