Exporter for blender can be found here: https://github.com/stig-atle/io_scene_pbrt

Either clone the repository and place it in the blender addon folder, or download as zip and install through the addon menu from the following url: https://github.com/stig-atle/pbrt-v4/archive/master.zip

For more information about the exporter and how to use it - visit the repository.