PDFView, my trusty MacOS PDF viewer is dead. But fortunately, Skim comes to the rescue. Skim has pretty much the same features as PDFView. For example, you can tell it to automatically reload a PDF file when it changed on disk, which is a needed feature if you are going to use LaTeX seriously. Also, though this is not without flaws, Skim allows you to annotate PDF files in fancy ways: to do so, it cheats since it does not actually modify the PDF file itself, but rather the file’s metadata… which means that if you share the PDF file, by email, the annotations will silently disappear. Fortunately, there are ways around it.
I have never understood why Adobe and others make it so difficult to annotate PDF files. This is really an historical mistake.