So I started using Calibre and it is excellent in collecting metadata with its search tools and organizing the data. For output of PDF I CAN USE the metadata plugin mapping to write out to the Description fields. Wonderful!

So what is missing? Why must I whine?
I use a Windows 10 machine (and previously 7, vista, etc.) along with a Android tablet. Over the years, I have collected metadata into the PDF books placed them into various places in the Description and created new Custom Properties. I write the metadata into the PDF file because the extra properties/metadata persist when transferred to non-NTFS formatted storage and then back to NTFS storage if it is kept in the file. I have many, many (5000+) PDF files.
With that background in mind, would it be possible to write out to Custom Properties metadata fields?