Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
Xournal is an application for notetaking, sketching, keeping a journal using a stylus. It is free software (GNU GPL) and runs on Linux (recent distributions) and other GTK+/Gnome platforms. It is similar to Microsoft Windows Journal or to other alternatives such as Jarnal and Gournal.
Xournal supports annotation of PDF files. Xournal uses the Poppler library to render PDF documents. The documents then become immutable background images. Annotation can then proceed using any of Xournal's standard tools: Pen, eraser, text, and highlighter.