Can I open the file in Photoshop, change the setting to CMYK and then save it as a PDF again, thereby converting the document to CMYK?
If you do that, the PDF will be unprintable. Separating the entire PDF in Photoshop will cause black text and other elements to separate to 4-color black--that is, the black text will have cyan, magenta, yellow,a nd black inks in it, which is not what you want. The PDF will also be "rasterized" (broken up into pixels), which means that the type and logos will not print smoothly.
You need to do one of three things:
1. Use Enfocus PitStop, Quite a Box of Tricks, or some other PDF tool to separate the images in the PDF; or
2. Have your client pull his head out of his ass, separate the RGB images to CMYK, place the CMYK images in Publisher, and create the PDF properly; or
3. Have your service bureau use in-RIP separation (if they have a Level 3 PostScript RIP) to separate the RGB images to CMYK. (This works, but does not exactly produce stellar results. Most in-RIP separation isn't the best.)
This would be a wonderful industry if it weren't for the clients... :)
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