.pbstream file

What is a pbstream file?

Pbstream file is a compressed protobuf file Cartographer creates when storing its sensor data. This file only contains what Cartographer claims to be useful sensor data, therefore this file is a lot smaller than its from the source(.bag file). This file's structured data has also been serialized with google's Protocol Buffers method for storing purpose.
Quote from google, "a cartographer state captured while performing SLAM on this sensor data".

Use

This file used only to recombine with the full sensor data file (bag file) and create a point cloud used for visualization. Either this can be of the format ply(3D) or pcd(2D). These files are then the high resolution map.

Sources

Protocol buffers method
Short text about pbstream