This is most probably a dump file created by either the EXP, or EXPDP utility. You can use IMP (if it was EXP) or IMPDP (if it was EXPDP) to extract DDL from these files.
I don't know how to distinguish the two formats, my advice would be to try to access the files using IMPDP first, and it that fails, using IMP. All of these utilities are described
in the documentation. It seems that in case of IMPDP, you can use the SQLFILE parameter to dump the DDL of contained DB objects. The IMP has a similar SHOW parameter.
There are usually log files created during import/export, if you had the import log file, you'd see what was exported and when. Perhaps you could obtain the log from the same source you obtained the DMP from?
