PDF was introduced by Adobe in 1993, made an open standard in 2008 (ISO 32000), and has gone through numerous revisions of both the main document format and for the associated metadata. The most recent update, version 2.0† (ISO 32000-2:2020) is illustrative of this evolution, with several features added and depreciated†.
Features added include† wider support for text encodings, rich media inclusion, improvements for representing document structure in a ‘Tagged PDF’ and more. Features depreciated includes the use of XFA forms, Flash/Shockwave support, XObjects and use of the document information dictionary.
There have also been servers versions of PDF introduced: PDF/UA (ISO 14289) ‘Universal Access’ for accessibility†, PDF/E (ISO 24517) for engineering data, particularly geospatial, construction and manufacturing workflows†, PDF/VT (ISO 16612-2 and ISO 16612-3) for Variable data and Transactional printing† and PDF/X (ISO 15930) for prepress, which disallows interactive elements†.
Particularly relevant for the work at hand is likely PDF/A which is is an ISO-standardized PDF version with a focus on archiving, released 2005. Because of this focus, the format does not allow for external data (such as fonts), encryption, audio or video embedding, scripting or metadata embodied using ‘XFA’ (XML Forms Architecture) to be included in the document.
Converting other versions to PDF/A has been shown in tests to introduce errors of around 11%† which an approach which presents the metadata in an archival friendly manner at the point of production, as Visual-Meta would do. Furthermore, Visual-Meta in any version of PDF would afford a greater level of archivability for the elements of the document contained in the document and not refered to externally.