Recommended and mentioned products
MuPDF is a lightweight PDF and XPS viewer.
Skim is a PDF reader and note-taker for OS X.
I would like to plug the open source macOS PDF reading/annotations application Skim. It is the most proficient scientific document viewer I have ever used. Here is a link to it, I have no affiliations to the application: https://skim-app.sourceforge.io/.
Okular is a universal document viewer based developed by KDE.
Yes, exactly, visual things drawn on top of the PDF. I mean things like Okular has: highlighting text (like with a yellow highlighter on paper books), freeform draw tool (like scribbling with a pen), adding short text comments and longer notes, etc. To illustrate, the annotations in an old Okular version looked like this: https://docs.kde.org/stable5/en/okular/okular/annotations.html (the yellow note box, the...