There's a "pdftk" thing that provides a command-line "pdf merge" tool, but it won't install on my Mac for boring reasons. I found this tip which gives a commandline way to do it without installing anything:
gs -q -dNOPAUSE -dBATCH -sDEVICE=pdfwrite -sOutputFile=merged.pdf \ source1.pdf source2.pdf source3.pdf etc.pdf
Handy