Many times, my assemblies are too complex, or erroneous from failing imported surfaces, to successfully make a 3D PDF. I try to create it, and it either crashes SWx or it crashes Reader once it is opened, or sometimes both. Further attempts to open the saved 3D PDF file with Reader also crash Reader.
Here's how I get around this to create a useful 3D PDF for fab & assembly shop reference and sometimes clients.
First, save assembly as a Parasolid Binary. Components will need resolved beforehand, and I strongly recommend you do not resolve hidden or suppressed components or they will appear in the result.
Then, close the assembly. Open the Parasolid Binary file which was created. Assign it an assembly template and part template, which matters very little. Save that imported assembly as a 3D PDF.
This process destroys any FM tree structure of folders, and I believe also strips the components of rich custom properties.
However, it produces a 3D PDF which does not crash Reader, and can be inspected, measured, and its view can be modified to hide or make transparent selected components, create section views, etc.
This is sort of covered in one user's post from 8 years ago, but it deals with a slightly different method to solve it.
Note: I am not using MBD. MBD may have other options available which I am unaware of.
Ref. installations used: Windows 7 Pro 64, SWx 2018 SP2, both Adobe Reader DC and Adobe Acrobat Pro X.