Hello!
In the age of remote learning/teaching I started using Edge to draw in .pdfs I’m showing to my students and it really fits my basic need, but something more focused on the task and in spirit of FLOSS would great. What’s most important to me is ability to draw and easily erase what I drawn; handy page switching. More would be nice ofcourse.
I couldn’t find anything like that, and software like Krita or GIMP doesn’t work well for my needa, that’s why I’m using Edge.
If you want to do this to use it as a virtual whiteboard and/or teach remotely, you can use OpenBoard.
Otherwise, xournal++ as suggested by @drop is what I use for PDF annotation. You can use a tablet like the Huion H420 or Gaomon S620 for precise input on Windows or Linux.
I got Wacom Intuos few years ago as present. So now while I’m explaining something I can draw whatever I need in the moment, to get the idea through.