Okular is not only a great viewer for documents, but it can also be used to annotate and take notes on these documents. This is typically used with PDF documents.
$ sudo apt install okular-extra-backends
To annotate, open the document and choose Tools -> Review. The keyboard shortcut for this is
A sidebar appears with buttons to add popup notes, freehand, highlight and other operations.
Single-click on any of these buttons, to be able to perform that operation once. To perform it again, you will need to click the button again.
Double-click on any of these buttons, to be able to perform that operation multiple times. Press
Esc to disable the operation after you are done.
To edit the color, thickness or any other property of these annotation tools, right-click anywhere in the sidebar and choose Configure Annotations.
To remove any annotation (even from a PDF file), right-click on it and choose Delete.
To move annotations such as popup notes, hold Ctrl key while you click and drag them with the mouse.
By default, the annotations are saved locally in a hidden file in the home directory.
To save the annotations along with the file, save it as a Okular document archive. To do this, choose File -> Export as -> Document Archive. This is typically saved with the file extension
Reference: Annotations documentation page about Okular
Tried with: Okular 4.13.3 and Ubuntu 14.04