It is useful to turn layers of information on or off in the PDF file that you create, so that you have more control over what you see or print from the PDF file. You can create layers in your PDF by turning on embedded layer support, which is a printer property, when you print to a PDF.
Each map and browser frame becomes a layer in the PDF file. If you added a legend for a map, then it saves to a single layer in the PDF file.
When you create a legend in a Layout window, each layer on the map is represented in its own legend frame. This gives you a great deal of flexibility for designing your layout, because you can rearrange the order or placement of each legend frame. When you print the layout to a layered PDF, the legend frames save to a single layer in the PDF. You can have more than one map in a layout, so for each map you will see a legend layer in the PDF.
To print to a layered PDF file:
When you open the PDF file, all layers are shown unless you choose to hide some of them.