Hi! My kid needs to use some special software due to some underlying issues. I was looking into how it works and it basically loads a PDF version of his books and then just allows him to digitally use the exercises by adding text, drawing, etc. It also has TTS functionality but we don't use it.

As the annotation is the most important part of this software, I'm looking for something similar, basically non-destructive overlaying of a PDF, and the ability to edit these overlayed annotations at a later time, removing or changing answers given in the digital textbook.

They are currently using software that is in general use, but it's licensed and calling it "janky" is wording things friendly. I'm not looking to replace this software, just doing some research.

So, to summarize, I'm wondering if there already is a FLOSS program that allows you to annotate a PDF (not edit! the contents that are already in the PDF may not be changed) by adding text, drawing lines or shapes, etc.