How to export layers into multi-page PDF in Adobe Illustrator

You can’t. You have to faff around with artboards.

But… There is a quick-ish way to do it if you have Indesign.

1. Drop your multi-layered Illustrator file into InDesign (Ctrl+D)
2. After positioning image on a page
3. Make a duplicate (alt + drag with the mouse on Pages panel)
4. Right click on image
5. Select manage layers (对象图层选项 duì xiàng tú céng xuăn xiàng)

Fin.

Solution found here (Thanks Creather).