Both .cdxml and .cdx files come from a software called ChemDraw, so it's natural this would be the software that might be used for cdxml to cdx conversion. This looks to be some kind of data exchange export for the purpose of sharing data between two ChemDraw users.
Alas, we have not found any useful information about such export, so if you need specific help or have particular issue, it would be best to contact either ChemDraw support or to reach out to ChemDraw's online community.