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 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.
Pro Free and open-source (GPL license)
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.
Con Written in Java using Swing toolkit
As with most Java apps, the UI looks alien to all operating systems.