Preparing Your system
sudo apt-get install gs pdftk
Now we will see one example how to combine pdf files
let us assume we have 1.pdf,2.pdf,3.pdf files and now we need to cobmine this files using the following command
gs -dNOPAUSE -sDEVICE=pdfwrite -sOUTPUTFILE=combinedpdf.pdf -dBATCH 1.pdf 2.pdf 3.pdf
the output file will be combinedpdf.pdf
Rename “1.pdf 2.pdf 3.pdf” to whatever your file names are (separate your file names with a space).
If you want to know more options available for gs command check main page