Concatenate pdf files in Linux with GhostScript, thanks to http://doeidoei.wordpress.com/2009/04/12/easy-way-to-concatenate-pdf-files-in-ubuntu-linux/
gs -q -sPAPERSIZE=letter -dNOPAUSE -dBATCH -sDEVICE=pdfwrite -sOutputFile=output.pdf file1.pdf file2.pdf file3.pdf [...] lastfile.pdf