OCG layers in PDF files can appear merged into one object when the file is opened in Illustrator. Here’s the short, non-technical version.
What is an OCG?
OCG stands for optional-content group – the PDF standard’s name for a layer. Many design and prepress tools write spot colors, cut lines, varnish and notes as separate OCGs so they can be turned on and off. Before extracting anything, check the Layers panel in Acrobat first.
What Illustrator does with them
When Illustrator opens a PDF, it does not import OCGs as editable layers. It flattens the visible content into a single layer. The information still exists in the original file – Illustrator simply doesn’t expose it.
That’s why a viewer toggle (hide/show a layer) isn’t the same as extraction: toggling visibility never gives you separate, named files you can actually work with.
The fix
Read the OCGs directly and export each as its own PDF, then bring those into Illustrator as individual layers. This is the practical way to recover usable groups from an inherited PDF without rebuilding them by hand.