How can I combine multiple PDFs using the command line?
Have a look at "Combining PDF files on the command line in OSX" in Joining PDF Files in OS X From the Command Line.


  It turns out that from Tiger onwards, OSX ships with a Python script
  that does exactly what you need. The script is already executable, and
  Python is pre-installed on OS X, so all you need to do to run it is
  opening the Terminal and typing


"/System/Library/Automator/Combine PDF Pages.action/Contents/Resources/join.py" -o PATH/TO/YOUR/MERGED/FILE.pdf /PATH/TO/ORIGINAL/1.pdf /PATH/TO/ANOTHER/2.pdf /PATH/TO/A/WHOLE/DIR/*.pdf


Also on the linked page it suggests making a symbolic link for the join.py file to make typing easier however they omitted the -s in ln -s ... ..., and without it, a hard link is created.  Probably wouldn't matter, however though I'd mention it.