I wanted to combine some PDFs, but apparently pdftk is not compilable on Mac OS Sierra.
Error: pdftk currently does not build on OS X 10.11 or greater.
Error: See https://trac.macports.org/ticket/48528
Error: Failed to fetch pdftk: incompatible OS X version
But this trick works.
/System/Library/Automator/Combine PDF Pages.action/Contents/Resources/join.py -o combined.pdf 1.pdf 2.pdf