I am trying to export a drawing as a PDF file and I have discovered that certain parts of the drawing although visible as a model or in sheet mode are not visible as a PDF file.  I don't even get a preview. I have checked layering and all the layers are turned on and are not frozen.  I have noticed it is certain blocks ( such as doors ) that do not convert. I have exploded them back into line segments without luck.  I checked an Auto-cad users sight  where people have reported the same issue. Some power users in the know point the finger at Adobe, some at Auto-desk, and others at Windows. Any thoughts in the Draft Sight community as to what is going on? Thanks in advance