Combine PDFs with Ghostscript

To combine PDFs in the terminal, use Ghostscript:

gs -q -dNOPAUSE -dBATCH -sDEVICE=pdfwrite -sOutputFile=merged.pdf source1.pdf source2.pdf source3.pdf

Taken from this StackExchange answer: https://apple.stackexchange.com/a/293198/77352