You need to go File > Export. Then select PDF. In the PDF export settings you'll have to select the 'Output' tab. Here it becomes important what you select (depending on the colors you want in your resulting PDF). For CMYK only, select 'Convert to Destination (Preserve Numbers)'. For 'Destination', select something like 'Document CMYK - ISONewspaper26v4'. ISONewspaper26v4 is a standardised color space for Newspaper printing in Europe. After choosing all that, click 'Export'. Your resulting PDF will now only contain CMYK objects.
A few things you need to be aware of:
- Don't choose just anything for PDF presets, but rely on standards like PDF/X-1a (available PDF preset in Adobe InDesign). BTW: colors will always be converted to the CMYK if you use these presets (next to some other features that will be set correct automatically for you).
- Make sure to select a proper destination output profile. As I said, ISONewspaper26v4 could be ok for Newspaper printing, if you're located in Europe. The Ghent Workgroup has a nice overview on their website to see which output destination profiles to select for which region: http://www.gwg.org/recommended_ICC_profiles.phtml. They also provide PDF presets you can download for exporting to PDF (also based on PDF/X-1 presets btw): http://tinyurl.com/7an6j4n. If don't have the ISONewspaper26v4 ICC-profile, you can download and install it first from this page (bottom): http://www.wan-ifra.org/articles/2011/01/17/iso-profiles-download.
And if all of this doesn't work, you can still ask your printer exactly what to do.
Hope this could help.
Oh yes, I see. But the chosen destination color profile doesn't need to have CMYK in it's name in order to give you CMYK output. Try any of the ICC profiles from the category (devided by black line in pull down menu) beginning with 'Coated FOGRA27', 'Coated FOGRA39', etc., and you will see you'll have CMYK output in the end (all ICC profiles from this categoy are CMYK based output profiles).