Jarnal is an open-source application for notetaking, sketching, keeping a journal, making a presentation, annotating a document - including pdf - or collaborating using a stylus, mouse or keyboard.
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Pro Free and open-source (GPL license)
Pro Collaboration over network (although limited)
It is possible to connect multiple Jarnal clients to a single Jarnal server for collaboration on the same document. However, only one person can make changes at a time.
Con Written in Java using Swing toolkit
As with most Java apps, the UI looks alien to all operating systems.
Con Some actions are not intuitive
Opening a PDF into Jarnal is not intuitive. The only way I found is by using the supplied shell script. I could not find how to do it from within the UI.