After 1.5 years in the making, @gnome Papers 50 will let you create freestanding text & ink annotations on PDFs (using a Wacom tablet stylus, for example), i.e. drawing & handwriting (to circle things or sign documents the old-fashioned way, or highlighting on scanned documents): https://lbaudin.pages.gitlab.gnome.org/blog/posts/drawing-writing-with-papers/
Used it in practice today. I've been hoping for this for about 20 years.
Papers nightly is such a massive improvement!
Thank you @lbaudin for implementing it 
