Hi eric - I guess that in Rhino if you go to Dimension menu > Dimension properties , and then expand it go to the Default entry in the options dialog that pops up, there is a setting there for "Text alignment" and one of the choices is "Horizontal to view", which should make the dimension text label face towards the reader which may help with what you want. The only problem is that the size of the text is fixed in world units so in a perspective view if the dimension text is further away from you it is smaller in size instead of being a fixed size on the screen.

Other than that you might want to take a look at SketchUp, it has a pretty simple system for doing annotations and it makes dimensions that face you and have consistent text sizes as well.

For generating 3D PDFs, you might want to look at Simlab Composer:
http://www.simlab-soft.com/3d-products/simlab-composer-main.aspx
but I don't know if it is possible to generate dimension annotations that will show up in the PDF viewer or not.

- Michael