It is documented that PDF only supports 2D graphics so I would use either a high resolution or Sander's conversion to 2D graphics:
from the PDF Export Documentation it says:
Stores text, fonts, images, and 2D vector graphics in a device- and resolution-independent way.
Also, I found in the past that the export resolution depends on the original size of the graphic and that ImageResolution->Infinity undoes this. so I would fix the original size and then rasterize it with:
g = Graphics3D[{Line[{{-2, 0, 2}, {2, 0, 2}, {0, 0, 4}, {-2, 0, 2}}]}, ImageSize -> Large]
Export["out5.pdf", g, ImageResolution -> 1200]
You can move the 1200 up or down to suit your needs/ file size.
Regards,
Neil