The monitor uses RGB colors. The printing company works with CMYK colors.
Monitors are able to show colors that the printing company cannot print. This refers primarily to the vivid colors. If you wish to see how the document would look like after printing, then the PDF has to be converted to CMYK.
The difference between RGB and CMYK cannot be observed in all cases.
When to use it?
If RGB colors are in the PDF, and you wish to convert them to cmyk.
What part of PDF is affected by it?
The color spaces of images, fonts, graphical elements get modified. The embedded ICC profiles get deleted also usually in case of rgb – cmyk conversion. The Pantone colors and other separated colors do not change.
What to watch for?
This conversion affects the entire document, therefore, it is important to check the document after conversion.