.osm.pbf it`s the extension for OpenStreetMap extract that I get from a site that takes daily extracts from osm: http://download.geofabrik.de/
How to import these files in QGIS? On Linux or Windows? The more simplest variant, if it exists.
QGIS is now able to read .pbf files with
Add vector layer, but the data size will most probably kill QGIS with most of the available files.
ogr2ogr -f SQLite my.sqlite some.pbf in the OSGEO4W shell to convert the pbf into a spatialite database, and connect to that in QGIS.
the openstreetmap plugin (current version 0.5) only reads osm-files , but there is a commandline-convert (both for linux as well as for windows. I only tried windows), which can convert *.pbf to *.osm. these converted osm can be importet into qgis via openstreetmapplugin
url for converter: http://wiki.openstreetmap.org/wiki/Osmconvert
You can also get the shapefile versions of the extracts. That way you won't have to convert anything.