3D models in PDF documents

PDF can do a lot of things. One them is embedding 3D models in the file and displaying them. The user can orient them freely in 3D space and even choose how they should be rendered (wireframe, solid, etc). The main use case for this is engineering applications.

Supporting 3D annotations is, as expected, unexpectedly difficult because:

1. No open source PDF viewer seems to support 3D models.
2. Even though the format specification is available, no open source software seems to support generating files in this format (by which I mean Blender does not do it by default). [1]

But, again, given sufficient effort and submitting data to not-at-all-sketchy-looking 3D model conversion web sites, you can get 3D annotations to work. Almost.

As you can probably tell, the picture above is not a screenshot. I had to take it with a cell phone camera, because while Acrobat Reader can open the file and display the result, it hard crashes before you can open the Windows screenshot tool. 

[1] Update: apparently KiCad nightly can export U3D files that can be used in PDFs.