How to downsample images within PDF file?

Here’s an example of how you can downsample all (color, gray and mono) images to 72dpi with a Ghostscript commandline:

gs \
  -o downsampled.pdf \
  -sDEVICE=pdfwrite \
  -dDownsampleColorImages=true \
  -dDownsampleGrayImages=true \
  -dDownsampleMonoImages=true \
  -dColorImageResolution=72 \
  -dGrayImageResolution=72 \
  -dMonoImageResolution=72 \
  -dColorImageDownsampleThreshold=1.0 \
  -dGrayImageDownsampleThreshold=1.0 \
  -dMonoImageDownsampleThreshold=1.0 \
   input.pdf

Update:
The *ImageDownsampleThreshold=1.0 parameters enforce that all Images with a resolution higher than the target resolution of 72 dpi will be downsampled. If this parameter is not given (or set to a different value), the default values will be used: *ImageDownsampleThreshold=1.5. This default value will only downsample images with a value of 108 dpi (or higher) and leave the other ones untouched.