I am looking for an iPad app (Android app references will also be considered useful) which allows reading PDF files and annotating them with a stylus, resembling the experience I would have with a pen and a piece of paper as closely as possible.
Not enforcing to go through button clicking, context menus or popups every time I want to write something. (i.e. add annotation, accept annotation). It completely breaks the reading experience.
I don't even care if that kind of a step-based work-flow is better if you want to edit/delete the annotations. I would be happy even if deletion/edition was not allowed, a good old cross-out would be enough for me.
The writing experience should be snappy and responsive. I don't want lines to follow my stylus-tip as I write, like laggy comets in orbit. A good example of good behaviour in this regard is Wacom's Bamboo Write App
Optionally it would be simply awesome if it was had handwritting calculator capabilities like MyScript Calculator. I am used to make calculations in the margins, when reading engineering or math-related books.