It is very simple to merge serveral PDF documents together into one PDF document. Just make sure that the gs package is installed on your system.

::::console
[email protected]:~$ gs -dNOPAUSE -sDEVICE=pdfwrite \
> -sOUTPUTFILE=result.pdf \
> -dBATCH one.pdf two.pdf three.pdf

This command will merge the thee PDF documents one.pdf, two.pdf and three.pdf into the resulting result.pdf document.