The command-line tools are provided by sumatrapdf-tool, which is installed next to SumatraPDF.exe. They only work after SumatraPDF has been installed.
Available in pre-release 3.7
Usage: sumatrapdf-tool trim [options] <input filename>
A PDF page has a fixed dimension. It can also have MediaBox, CropBox, BleedBox, TrimeBox and ArtBox which define rectangular area within the page.
You can see what kind of boxes a PDF page has using sumatrapdf-tool pages.
Using trim you can trim the area outside of a given box i.e. if there is content outside of the box, it’ll be removed.
With -e it’ll do the opposite i.e. remove the content inside the box.
The -m margin can be given as a single value applied to all sides, two values <V>,<H> (vertical, horizontal) or four values <T>,<R>,<B>,<L> (top, right, bottom, left). Positive values move inwards, negative outwards.

Examples #

Trim to the MediaBox with a 200 point margin inwards:
sumatrapdf-tool trim -b mediabox -m 200 -o out.pdf in.pdf
Trim with 20 points off the top & bottom and 30 points off the left & right:
sumatrapdf-tool trim -b mediabox -m 20,30 -o out.pdf in.pdf
Trim with separate margins for each side (top, right, bottom, left):
sumatrapdf-tool trim -b mediabox -m 10,10,50,20 -o out.pdf in.pdf
Remove the contents of the ArtBox instead of keeping them:
sumatrapdf-tool trim -b artbox -e -o out.pdf in.pdf

All options #

Usage: SumatraPDF trim [options] <input filename>
  -b -    Which box to trim to (MediaBox(default), CropBox, BleedBox, TrimBox, ArtBox)
  -m -    Add margins to box (+ve for inwards, -ve outwards).
                  <All> or <V>,<H> or <T>,<R>,<B>,<L>
  -e      Exclude contents of box, rather than include them
  -f      Fallback to mediabox if specified box not available
  -o -    Output file