title: Converting (any) PDF to black (K)-only CMYK
body: <p>This is related to:</p>
  
  <ul>
  <li><a href="https://stackoverflow.com/questions/6241282/converting-pdf-to-cmyk-with-identify-recognizing-cmyk">Converting PDF to CMYK (with identify recognizing CMYK).</a></li>
  <li><a href="https://stackoverflow.com/questions/3418268/script-or-some-other-means-to-convert-rgb-to-cmyk-in-pdf">Script (or some other means) to convert RGB to CMYK in PDF?</a></li>
  </ul>
  
  <p>... but a bit more specific here: say I have an RGB PDF, where the text color is "rich black" (R:0 G:0 B:0 gone to C:100 M:100 Y:100 K:100), and diverse images and vector graphics. </p>
  
  <p>I would like to convert this to a CMYK PDF, using a free command line tool (so it is batch scriptable under Linux), which </p>
  
  <ul>
  <li>has contents <em>only</em> in the black (K) channel:
  
  <ul>
  <li>Preserves vector graphics (+ text glyphs) - colors become grayscale in black (K) channel only</li>
  <li>Images get converted to grayscale in black (K) channel only</li>
  </ul></li>
  </ul>
  
  <p>Thanks in advance for any answers,<br>
  Cheers!</p>