Snippet - Convert PDF RGB to PDF CMYK Fogra39

You need to download Coated_Fogra39L_VIGC_300.icc from here.

Then, run:

gs -dSAFER -dBATCH \
    -dNOPAUSE -dNOCACHE -sDEVICE=pdfwrite -dAutoRotatePages=/None \
    -sColorConversionStrategy=CMYK \
    -dProcessColorModel=/DeviceCMYK -sOutputICCProfile=Coated_Fogra39L_VIGC_300.icc \
    -sOutputFile=output_cmyk.pdf \
    input.pdf

Sources: