Anoto Group AB is a Swedish technology company which has invented, developed and licensed a product line of digital pens and a product suite to manage information capturing utilizing these (marketed as Anoto) (Source)
I recently developed a software that allows connecting such a PEN to an Android tablet using HID:
The software uses the HID protocol over bluetooth and implements the custom communication protocol required for getting the coordinates and the pressure data from the digital pen.
This way pages of text can be store in the internal memory of the PEN, or transferred wirelessly to a mobile computer for storing or further processing. There are multiple possibilities here: direct paper to technology path (for drawing a diagram and then interacting with it on the mobile phone/tablet) , on site calculations (including length, perimeter, etc), but also interactive sessions, such as group projects or even classroom lessons, as show in this video:
I cannot disclose the exact details used for communication, but I can say that the HID protocol used by Anoto comes with additional commands used to access and control the data processed by these pens. A very well organized structure, that allows downloading stored data or handling real time data, stats on memory or battery, and handwriting details such as pen pressure or coordinates .