====Merge====
  pdfunite in01.pdf in02.pdf in03.pdf out.pdf

====Resize====
  gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/ebook -dNOPAUSE -dQUIET -dBATCH -sOutputFile=output.pdf input.pdf

in dPDFSETTINGS : screen, ebook, prepress, printer