• brew tap FLEWID-AB/homebrew-pdfjam
• brew install pdfjam
• You need MacTex. Download it here https://tug.org/mactex/mactex-download.html
• After installation of MacTex, you need to export your pdflatex path. You can find the executable by searching for it. sudo find -name pdflatex. For me it was /usr/local/texlive/2017/bin/x86_64-darwin
• nano ~/.bash_profile then add the path you just got from search. example: export PATH=/usr/local/texlive/2017/bin/x86_64-darwin:$PATH and . ~/.bash_profile.

After that you should be able to use pdfjam.