The PDF viewer has a draw tool which allows you to write on top of the document. This works well for the most part but has two bugs i've found
The biggest problem with it is when you finish drawing and alt tab, the things you've written so far will merge into a moveable box, which makes it impossible to write within that box and to edit any of the text. This means that you could be solving some equation, move somewhere down the page to underline something, and after alt tabbing all the drawn lines will get put into a box which will lose you all the space and progress. You cannot undo this merge with ctrl Z, and frankly I don't see why you'd ever want to have things merge into a moveable box anyway.
The second issue I found is minor, the colour picker doesn't change the colour of the line.
Hope this gets fixed because it's very unusable in it's current state cheers
