It might be better to use the TransformColorSpace of MagickImage to change the image from RGB to CMYK:
And if your PDF file contains multiple pages you should use the MagickImageCollection instead and then call TransformColorSpace on each of the images in your collection. And you might want to change 'settings.Density' to a DPI of 300 to get an image with a better quality.
image.TransformColorSpace(ColorProfile.SRGB, ColorProfile.USWebCoatedSWOP);