How to concatenate several PDF files

Use Ghostscript. There is no Mac package, but a ./configure --without-x && make && sudo make install will do. Ghostscript offers a wealth of functions to generate or manipulate existing PDFs, check for details.
So, to concatenate: gs -q -sPAPERSIZE=a4 -dNOPAUSE -dBATCH -sDEVICE=pdfwrite -sOutputFile=out.pdf in1.pdf in2.pdf ...

Bad Behavior has blocked 85 access attempts in the last 7 days.