I finally did as follows:
- Renamed the symlink file with a '_x' suffix.
- Copied the actual file /opt/local/libexec/cups/backend/cups-pdf over to the folder /usr/libexec/cups/backend/.
- Typed in sudo chown root:wheel /usr/libexec/cups/backend/cups-pdf.
[crarko adds: I haven't tested this one. It's strange that the symlink should break like that. I suppose it's a permissions issue.]

