Two other useful programs are Skim, for viewing PDFs, and Mendeley, for organising PDFs of journal articles. Both can be installed with brew cask.

Installing and running Skim

This useful website provides instructions for installing Skim via the command line, using brew:

brew cask install skim

Then, suppose you want to open the PDF ~/Downloads/paper.pdf. On the command line:

open -a Skim ~/Downloads/paper.pdf

Installing Mendeley

Similarly, this page provides instructions for installing Mendeley via the command line:

brew cask install mendeley