I had to merge multiple PDF files into one recently and wanted a quick way to do it. In order to do this I had to
- Download and install ghostscript and pdftk
sudo apt-get install gs pdftk
- cd into the directory containing the files and issue command
gs -dNOPAUSE -sDEVICE=pdfwrite -sOUTPUTFILE=combined.pdf -dBATCH first.pdf second.pdf