You wrote:
“Why a secrect archive? What is the Vatican hiding?”
Nothing. Any scholar can see these documents. The patriarchs have secret documents too - and no one gets to see them. Usually those are letters and documents about internal problems, bad priests, bad marriages, etc. The Catholic Church also has plenty of such documents. The one muentioned in the article are not actually secret at all, however.
Oops! Should be “ones mentioned”.