From that I understand you must make a "double copy" with "edges" of the volume for dimension it!

Seems only curves can have dimensions for export PDF 3D!

But from my site and also image above, i don't see why PDF 3D can't export the dimensions as you draw them!
(i have not tested PDF 3D export because I have only DSM 1 )



PS you can also try free Bentley viewer, it has export PDF 3D (not shown on the site but exists inside the prog ! )

https://www.bentley.com/en/products/product-line/modeling-and-visualization-software/bentley-view

And normally you can make dimensions in PDF 3D
https://helpx.adobe.com/acrobat/using/measuring-3d-objects-pdfs.html