$ gs -dNOPAUSE -sDEVICE=pdfwrite -sOUTPUTFILE=merged_file.pdf -dBATCH pdf_1.pdf pdf_2.pdf
http://www.linux.com/news/software/applications/8229-putting-together-pdf-files
$ gs -dBATCH -dNOPAUSE -q -sDEVICE=pdfwrite -sOutputFile=OUTFILE.pdf file1.pdf file2.pdf
CentOS6: Install pdfjam and use pdfjoin