Robert_Charlton - 8:16 am on Jul 20, 2013 (gmt 0)
Valuable Duplicate = Rare manuscripts, which are rare, but may happen in other places
To oversimplify... in duplicate situations, the site with the most PageRank wins. "PageRank" these days includes other PageRank-like "link juice" factors. Such factors (among others) are a measure of online popularity or reputation.
So... if you want to rank for the text of the documents, then you essentially need more relevant, high quality inbound links than competing sites offering the same text. In the past, I've seen Google return multiple copies of documents or articles if there's sufficient linking to the sites that publish them. In your situation, I can imagine that unique and useful commentary might be one way of attracting the kinds of links you need.
many articles, besides this G+, FB , feeds, etc....
Currently, this type of social promotion by itself is not sufficient to produce Google rankings. If it drives traffic to your site, then, over time, that traffic might generate links. You might say that inbound links of sufficient quality are in part what suggests to Google that your material is "authoritative".
It's likely that, say, the Morgan Library... if it offered this material online... would be likely to outrank you for the same queries because it has a big head start.
Keep in mind, though, that if the "rare manuscripts" are rare and not copyrighted, the text content is likely to become less rare when published online. That's generally why your site will need to offer... in addition to the manuscript content... unique self-generated material, to establish a genuine reputation online.