What are you trying to do with it?
.pbf is a compression format (a bit like a .zip-file) which contains the .osm-file. The .osm-file is nothing more then an XML-file with a specific format and a .osm
-extension.
Those are not meant for day-to-day users but more for developers.There are some programs able to open those files, such as JOSM or Osmosis.
An end user cannot open provided XML files with it