1.Open terminal.Go to directory where you put files.(Ex:cd Desktop/pdfs).
2.Here 1.pdf 2.pdf are two other pdfs(Desktop/pdfs).Combine two PDF files into one by typing below command in terminal.
gs -dNOPAUSE -dBATCH -sDEVICE=pdfwrite -sOUTPUTFILE=output.pdf 1.pdf 2.pdf
