Ref: https://stackoverflow.com/questions/2507766/merge-convert-multiple-pdf-files-into-one-pdf
gs -dBATCH -dNOPAUSE -q -sDEVICE=pdfwrite -dPDFSETTINGS=/prepress -sOutputFile=merged.pdf mine1.pdf mine2.pdf
pdfmerge() { gs -dBATCH -dNOPAUSE -q -sDEVICE=pdfwrite -dPDFSETTINGS=/prepress -sOutputFile=$@ ; }
Ref: https://askubuntu.com/a/256449
gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/screen -dNOPAUSE -dQUIET -dBATCH -sOutputFile=output.pdf input.pdf