You open a PDF in Illustrator. Acrobat showed you six layers a minute ago: spot colour, varnish, a cut line, the print artwork, some notes. Illustrator shows one. Everything is welded into a single layer, and the Layers panel offers nothing to expand.
Nothing is broken. Illustrator is doing exactly what it was built to do, and understanding that is the difference between an afternoon of redrawing and a few minutes of extraction.
PDF layers and Illustrator layers are two different things
The word “layer” is doing double duty here, and that is the whole problem.
An Illustrator layer is a container inside an .ai document. It has a name, a colour, a stacking order, and it exists only for Illustrator. When you save that document as a PDF with “Preserve Illustrator Editing Capabilities” switched on, Illustrator quietly embeds a private copy of the .ai data inside the PDF. Open that PDF in Illustrator later and it ignores the PDF part entirely and reads its own private copy back. That is why your own PDFs usually reopen with layers intact.
A PDF layer is something else. The PDF specification calls it an optional content group, or OCG. It is a way of tagging drawing instructions so a viewer can show or hide them as a set. Acrobat, most RIPs, CorelDRAW, AutoCAD, InDesign and a long list of prepress tools all write OCGs. They are part of the public PDF format, not any one application’s private data.
Illustrator can write OCGs. It cannot read them back as layers. When it opens a PDF that has no embedded .ai data, it falls back to the generic PDF importer, and that importer draws every visible object onto a single layer, in page order, with the OCG tags discarded. The groups were real. They are simply not carried across.
Why the usual fixes do not work
People try the same things in the same order, and none of them touch the cause.
Reopening the file, or opening it with different import options, does not help, because the importer has no setting for OCGs. Placing the PDF instead of opening it gives you a linked image with the same flattening. Turning layers on and off in Acrobat before saving changes what is visible, not how Illustrator reads the structure. Saving from Acrobat to a new PDF preserves the OCGs, and Illustrator still flattens them.
Asking the sender for the original source file is the correct fix when it is possible. Often it is not. The designer has moved on, the agency has closed, the file was made in software the shop does not own, or the only thing that ever existed was the print PDF.
Check before you rebuild
The important question is not “why is Illustrator doing this” but “is the structure still in the file”. If the OCGs survived the export, there is something to recover. If the PDF was flattened before it reached you, there is nothing, and no tool will invent it.
The check takes thirty seconds and costs nothing. Open the PDF in Acrobat and go to View, then Show/Hide, then Side Panels, then Layers. If the panel lists named groups, toggle each one and watch the page. A cut contour that disappears when its layer is hidden is a real, separable group. A layer that changes nothing on the page is usually an empty or decorative group.
If the panel is empty, the layers are gone. Rebuild, or go back to the sender. If the panel is full and the toggles work, keep reading.
Getting the groups out as files you can use
Seeing a layer in Acrobat and having it as an editable file are not the same thing. Visibility toggles never give you a separate object you can place, recolour or send to a plate.
The way through is to read the OCGs directly from the PDF and write each one out as its own PDF, keeping the original page size and position. Illustrator opens each of those single-group PDFs cleanly, because each one contains only one group and there is nothing left to flatten. Stack them back in one document and you have the separation the original file was carrying all along: spot colour on its own layer, varnish on its own, cut line on its own.
This is what PDFLayerExtract does. You upload the PDF and it lists every group it can find, by type, before you pay anything. If the groups are there, the paid package gives you one PDF per group plus a small Illustrator script that imports them as named layers in one step. If the groups are not there, the free check tells you so and you have lost nothing but half a minute.
What this will not do
It will not rebuild the original .ai file. Text that was outlined stays outlined. Effects that were flattened stay flattened. Object names, appearance attributes and the designer’s original layer hierarchy do not come back, because they were never in the PDF to begin with.
What comes back is the production separation: clean, named, correctly positioned groups you can work with. For most inherited print files, that is the part that was costing the hours.