RGB (red, green, blue) color mode is for screen displays, computer screens, TVs, smartphones, etc., while CMYK (cyan, magenta, yellow, black) color mode is for printing.
While there are more colors available in the RGB mode and they can be more vibrant than colors in the CMYK mode, at some point, they have to be converted to CMYK (process) to print. Ideally, all graphics, images, text, etc. should be converted or created in the CMYK mode before PDF files are sent to print. While the RGB image may look pleasing on a computer, that does not translate once the image has been converted to CMYK to print. There can be a definite visual difference in a color when it is converted from RGB to CMYK, so it is best that it is done before sending the files to the printer.