pdfunite in01.pdf in02.pdf in03.pdf out.pdf
gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/ebook -dNOPAUSE -dQUIET -dBATCH -sOutputFile=output.pdf input.pdf
in dPDFSETTINGS : screen, ebook, prepress, printer