Annotating PDFs in Okular on Linux
The standard PDF viewer on Ubuntu Linux is Evince. Evince allows for simple annotation in the form of text highlighting and adding of text comments. There is an "annotate" button on top of the window which enables annotation mode. However that may not be enough for presentations or marking students' work. In order to do that a better tool is Okular which is packaged for most Linux distributions.
Okular has more extensive annotation capabilities, via
Tools -> Review.
In full-screen presentation mode (eg when screen sharing a presentation on MS Teams), it has simpler hand-drawing tools to circle things etc. You can open the PDF, go to
View -> Presentation
and then drag the mouse to the top of the window which will open an annotation toolbar and allow you to annotate it.