execserver:
      ...
      waitgroups:
        a:
          processes: 2
      services:
        # this service allows to execute arbitrary binaries
        exec:
          # choose a waitgroup which can take heavy tasks. Chromium on
          # Linux needs about 500 MB to operate. Each parallel produce PDF
          # will take that memory.
          waitgroup: c
          commands:
            exec:
              env:
                # defaults to "chromium"
                - SERVER_PDF_CHROME=chromium # change to Chrome or absolute paths