New #RStats {xmpdf} 0.3.1 now allows you to read/write arbitrary pdf documentation info dictionary (DOCINFO) entries beyond the eight standard entries (although certain non-standard keys may be discarded with a warning depending on backend limitations).

https://cran.r-project.org/package=xmpdf

https://opensource.adobe.com/dc-acrobat-sdk-docs/library/pdfmark/pdfmark_Basic.html#document-info-dictionary-docinfo

https://trevorldavis.com/R/xmpdf/dev/reference/edit_docinfo.html#known-limitations