W51 documents were in the past created in old Corel WordPerfect and are probably not supported outside the original program.
It may be possible that the latest Corel WordPerfect still supports the import of old .w51 files and could re-save them to something more modern (like MS Word .doc files), but we have no exact information about this. You can always try to download and install trial version of Corel WordPerfect and see if you are able to open the documents yourself.