Gimp Python plugin to monochrome pdf and reduce size
On windows, place it in %APPDATA%\GIMP\2.XX\plug-ins or in C:\Program Files\GIMP 2\lib\gimp\2.0\plug-ins
To use the plugin use the item in the File menu.
[Configured at: Edit > Preferences > Folders > Plug-ins]
Tested on windows, but should work on linux.