Hello William,
According to my understanding you are using FIG2U3D function and are still not able to convert it in 3D pdf.
I got a YouTube tutorial for the same: (884) Cómo convertir plots 3D de Matlab a pdf - YouTube
If this doesn't work try using a slightly newer repository from here: GitHub - DmytroSytnyk/fig2u3d: Export MATLAB figure as U3D file or directly to 3D interactive PDF