Adding, parsing and extracting comments in a PDF file

I am looking at a way of making text comments on a PDF file, so that it is possible to:
1. see on which piece of text a comment is
2. make a clear visual difference between comments and the PDF file itself
3. easily navigate through comments (go to next/previous comment)
4. save the PDF file with comments, and allow other people to add comments to it
5. extract the list of comments to a text file, with some kind of info about the comment location

I tried Okular, it looks like the perfect tool, except for 5 for which I could not find a solution. But perhaps someone has a clue about this? Okular has also versions for major non-free OSes, which would have the nice bonus of allowing sharing these documents with people using those OSes.

I also had a look at xournalpp, but I could not find a dedicated "text comment" feature that allows adding comments that are visually easy to see, and that allows going from one comment to the next. So unless I missed something, xournalpp looks far less suitable.