Preparing Your system
sudo apt-get install gs pdftkNow 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
In the above command after "-sOUTPUTFILE=" type whatever name you want for the merged file (don't forget to append .pdf to the end of the file name). Rename "1.pdf 2.pdf 3.pdf" to whatever your file names are (separate your file names with a space).
After you run the above command, a new PDF file called combinedpdf.pdf will be created with your individual files merged according to the order you list.
If you want to know more options available for gs command check man page