There is already a similar question, which is about the Xoom and note taking. But I'd like to ask this in a more general way:
Given a tablet (and a pen), how can I write - not just adding notes and comments, more like one would write on paper - in a PDF?
The additional writing should be saved within the same PDF file and readable/showable by the most common PDF readers. Essentially I am looking for an Xournal alternative for Android.