Use sumatrapdf-tool.exe or SumatraPDF.exe with the same arguments.
Available in pre-release 3.7

In the application #

To compress a PDF in SumatraPDF:
  • open a PDF document
  • press Ctrl + K to open the command palette
  • select Compress PDF
Or:
  • open a PDF document
  • right-click to open the context menu
  • select Document > Compress PDF

From the command line #

To compress a PDF using SumatraPDF from the command line:
sumatrapdf-tool clean -gggg -e 100 -f -i -t -Z foo.pdf foo-compressed.pdf
This uses the most aggressive compression flags.
For an explanation of all flags, see sumatrapdf-tool clean.
If a PDF file is uncompressed, the compressed version should be smaller.
If a PDF is already compressed, this will have little effect.
You can also decompress a PDF.