As you may know, Xournal is an open-source application developed in GTK+, enabling the users to take notes, perform sketches and keep journals. Among others, it provides a pen, a highlighting feature, a text tool and a shape recognizer. Also, it has a special feature for performing annotations on PDF files (via the poppler library).
The latest version available is Xournal 0.4.8, which has been released a while ago, coming with a new feature for exporting to PDF, horizontal view mode, optional auto-save, enhanced icons for eraser and shapes buttons and bug-fixes.
Installation instructions:
Due to the fact that it is available via the default repositories on Ubuntu 18.10 Cosmic Cuttlefish, Ubuntu 18.04 Bionic Beaver, Linux Mint 19.x, Elementary OS 0.5 Juno and other Ubuntu derivative systems, installing the software should not cause too many problems.
All you need to do is update the local repository index and install xournal:
$ sudo apt update
$ sudo apt install xournal
Optional, to remove xournal, do:
$ sudo apt remove xournal