in PDFReader, see methods:
PDFReader.PDFSetStamp(), which enter/leave stamp status.
PDFReader.onTouchStamp(), which treat UI events.
PDFReader.onDrawStamp(), which draw stamp status.
in stamp status, it just record all coordinates, and Bitmap objects.
and draw recorded data to screen.
when stamp status leaved, all recorded data is added to Page, and then erase all recorded data.