[rmc]

For starters, the OSM PBF file format is not a protobuf file! Instead it's a collection of protobuf files inside each other!

You can read more in the fileformat: https://wiki.openstreetmap.org/wiki/PBF_Format

There are other problems, specific to OSM and not PBF/protobuf, like needing to store the locations of nodes until the end of file because they could be referenced anywhere in the file.