All above are correct. The problem you are having is your Layout scene is probably set to "last saved scene", instead of the appropriate saved scene in SU. When you generate a PDF in layout it automatically updates the reference, so if the last saved reference is not a section saved scene, then you get the pdf shown as it was last saved in SU.

Set your scene in SU with section, name the scene and save. In layout, in the Sketchup Model dialogue, select the same scene and render. Export PDF and it should show correctly.