There are numerous details that users might want to add as metadata into a PDF, beside the basic items usually allowed (such as Title, Author/Creator, Subject, Keywords, Description).
Examples might be:
- Number of pages
- Number of words
- Project manager
- Version number
There are several internationally respected standards for metadata, such as Dublin Core (in both the dc and dcterms namespaces) and PRISM (in several namespaces, such as prism, prism-ad, and pur).
There would also be innumerable in-house systems.
Currently it seems to be either very unwieldy or outright impossible to add metadata beyond the most basic fields (such as Title, Author/Creator, ...) into a PDF file, based on my experience with Adobe Acrobat, PDF-Exchange Editor, and one other application.
I don't really understand why it should be so difficult.
The main issue I can think of is whether an appropriate schema is available. That could add an extra level of bother for in-house conventions, or for little-known international 'standards'.
However, for the most popular standards, such as DC and PRISM, I really think by now the metadata supported by those standards should be able to be entered into a PDF without much hassle. Possibly involving a couple of drop-down menus:
Button 1: Add a new standard metadata field to this PDF.
Drop-down/Choice 1: Which schema/namespace to use? (Schema is version-specific. It might be easier to let the user choose the namespace, and then the software can default to the most recent (known) schema for that namespace.)
Drop-down/Choice 2: Which metadata name to include from an enumerated list.
Maybe custom metadata can be added too:
Button 2: Add a new user-defined/custom metadata field to this PDF.
*Display warning like: "May not be supported by all software." if appropriate.
Personally I also think it should be possible to edit the metadata in the XMP structure.
Of course, if it is free text entry, the user's changes would have to be validated by the software as being valid before being accepted. This may be why Adobe Acrobat (at version 7) only allows deleting entries from here, but not adding. Yet PDF-XChange Editor version 7 doesn't directly allow even deletions from the XMP structure listing.
Alternatively a structured dialogue could be created to ensure well-formed entries in the XMP structure. Indeed, that would partly be satisfied by the above buttons (but only for the case of adding new metadata fields, not editing or deleting existing fields).
(It is supposed that XMP files should be able to be imported to update the metadata, but that is (personally) my least preferred option, because it relies on some external method of creating an XMP file, which seems to be an arcane art. Maybe beneficial for some corporate/industry users.)