From time to time it is very useful to combine multiple PDF documents into one file.
Install gs (ghostscript) and pdftk (tool for manipulating PDF documents)
sudo aptitude install gs pdftk
Now we will see one example, which shows how to combine a few pdf files; 1.pdf,2.pdf,3.pdf.
gs -dNOPAUSE -sDEVICE=pdfwrite -sOUTPUTFILE=combined_pdf.pdf -dBATCH 1.pdf 2.pdf 3.pdf
If you want to know more options available for gs command check the gs man page