Is there a software for writing anywhere on the system with a pen/stylus on the touchscreen?

I'd like to write on top of my PDFs used for making presentations on my laptop with touchscreen. I do not need to save what I write, but I would like to circle or underline things, possibly write some equations, etc. and then erase them and move on.

Drawing tablets bring software of the sort I speak. Anything out there I could use with my Ubuntu 14.04? Thanks!

Since this question still comes up in the latest search results I want to let you know that it's possible to do more than one Freehand stroke with Okular.

You just have to double click the annotation icon (for example freehand stroke). This also works for the Highlight and Other Annotation Tools as well.

However, I still could not find out how to use the pen only for annotating and the touch screen for navigation through the document...

I 'll recommend you to try Okular. It gives you lots of such options to edit your pdf files. Open your pdf using Okular and press F6 or

Tools --> Review

It will show some tabs on the left side. Selecting these will enable you to do many functions such as underline, highlight, freehand line, ellipse, polygon, etc. Using freehand line option, you can write on your pdf. You can even edit these functions.

To install Okular, type these commands in your terminal:

sudo apt-get install Okular

Currently one of the best software I'm using in Ubuntu 20.04 LTS

Gromit-MPX gives me better exposure in online teaching to underline or draw something during class. I just have to press F9 and my mouse cursor becomes a pen and to reverse back again press F9.

It also has more functionality. It is also available in Ubuntu Store.