I own a Xerox Phaser 7800 and I get consistent results with registration black. I tend to use illustrator, so keeping my colors in the correct color is non-issue, but she uses Procreate and apparently can only export to PDF (I'm personally not familiar with the program).
Community Expert , This is a lossy conversion from a large to a rather small color space. On top of that your artwork might contain blending modes that simply do not in CMYK. So maybe you are asking because of a certain clipart file. This is why it won't work:
To take color matching in your hands, design within the CMYK color space and then convert to RGB yourself, without losing any colors. Or, if your software doesn’t support the CMYK color space, use an online RGB to CMYK converter to check which colors of your design are out of CMYK color space and adjust those. Tip!