The only way how to convert older WordPerfect W51 documents is to probably open them in a previous version of CWP that still supports it and print it as PDF using virtual printer. Another option would be to export it to some more common format like RTF, that could be opened in modern word processors.
Convert Corel WordPerfect 5.1 document file to Adobe Portable document format.
We found one software available in the database for this conversion.
Microsoft Windows software - convert w51 to pdf on Windows
Corel WordPerfect is word processor for Microsoft Windows.
Convert from w51 file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.