It would appear that the PLAIN format is SQL. To Quote the documentation:
pg_dump does not support all options for all backup file formats.
Particularly, to backup blobs the PLAIN format can not be used. Also,
a PLAIN file can not be interpreted, and can not be restored using
pgAdmin. The PLAIN format will create an SQL script that can be
executed using the psql tool. For standard backup and restore
purposes, the COMPRESS and TAR options are recommended.
I'm assuming you could restore using something like (on unix):
psql < backup-file
or from inside psql prompt (newdb is an empty db)
newdb=# \i backup-file
You might have to run as privileged user '
postgres' (on unix, say) or pass the appropriate credentials for the role to
psql so that it can create the db and roles and other entities indicated by the backup-file. The create database and role statements might not be present in the backup depending on whether it has the entire server (it has) or just one database (you should create the db and role(s)). So you'd have to create them before restoring it.