In some cases, pdf viewers will not show 3D content right away because they do not trust that it is safe. Adobe Acrobat will show a yellow bar at the top of the screen asking if you want to allow 3D content. To avoid having to do this every time in Adobe Acrobat, do the following:

  1.  Go to Edit > Preferences
  2. Click on the 3D & Multimedia tab
  3. Check the box that says “Enable playing of 3D Content”.
  4. Click OK.
  5. Click Options on the yellow bar at the top of the screen.
  6. Click Trust this document always
  7. Click the grey box in the center of the screen.
  8. Next time you open a 3d PDF you will not need to do this again

See the demonstration of this below for more information.