I don't have an iPad but I do a lot of free-drawing PDF annotation on Android with Squid. For maths it's way easier than typing Latex. Personally I wouldn't consider this kind of app unless it had free drawing as a primary tool. Even if you're focused on laptops, many laptops have pen input, especially those used by students. Many of my students use OneNote to annotate lecture slides with a pen; even though OneNote is pretty limited, there aren't many good competitors on Windows.