pointing to pdf of a volume

[eroux]

It could be useful to have a property to point to a pdf of a volume. The motivation is:

• some iiif manifests don't contain the link to the pdf although it exists and should be there (ex: BnF, IA)
• it's a useful thing to have for third party that only provide PDFs (although there's no current example of that, that may appear in the future)