What happens to overprinting process white (0% CMYK) objects when saving to AI with PDF Compatibility, Adobe®PDF or Normalized PDF from Adobe® Illustrator® CC or newer and viewing them in Acrobat® or ArtPro+?
Since the first Creative Cloud version of Adobe® Illustrator® you can find an option in Illustrator®'s Document Setup for .AI documents: Discard White Overprint in Output: to either discard the overprint tag of overprinting process white (0% CMYK) objects (default setting!) in the output file or preserve the overprint tag.
The discard white overprint in output feature will only be applied to the output file, so in Adobe® Illustrator®'s Overprint Preview, this setting is not applied and objects will still look overprinting.
When creating overprinting white objects in Adobe® Illustrator®, you get a warning message mentioning there are process white objects in your selection and that overprinting would only make sense in combination with transparency effects.
Adobe® Illustrator®'s Overprint Preview will still show the objects as overprinting.
In Adobe® Acrobat® Professional or Adobe® Reader when opening AI with PDF Compatibility or Adobe®PDF files
Preserve overprint (AI + PDF):
Discard overprint (AI + PDF):
In ArtPro+ when opening Adobe®PDF files.
In ArtPro+ when opening Ai files with PDF Compatibility
Preserve overprint: The object is set to overprint but the process separations are set to knock-out. If you would add a separation, that separation would be set to overprint.
When exporting Normalized PDF (both for preserve or discard white overprint in output) we will always discard the fill color completely, resulting in lineart with empty fills or strokes.