Yes I know, I can do it in Photoshop, Illustrator or even Vectorworks. The point is I use Layout to edit my plots. I don’t want to use, Layout and … to do what others do in one software.
I think it could be a pretty cool improvement of the soft.
maybe you can hack your way through this:
Create a plane over your model and apply as a texture the image from the pdf.
(if it is raster you can just copy it and save it somewhere, if it is vector you have to convert the pdf to raster image) scale it so that it fits your underlying model and adjust the material’s opacity.
Then create an ortho top view scene and export to layout. that way you can adjust the opacity in sketchup and you have the added benefit that you can update the image reference from the material texture.