How can I change the resolution of a PDF
You can use ImageMagick's scale or resize operators.


  ImageMagick® is a software suite to create, edit, compose, or convert
  bitmap images. It can read and write images in a variety of formats
  (over 100) including DPX, EXR, GIF, JPEG, JPEG-2000, PDF, PhotoCD,
  PNG, Postscript, SVG, and TIFF.


For example, from a terminal:

convert input.pdf  -resize 50% output.pdf


or

convert input.pdf -scale 50% output.pdf