I think that is a very confusing way to look at things. It's not that we collectively agree that the document means that something exists, but rather this is the category of things for which existence is by definition caused by the existence of those documents. A corporation is not some ethereal thing that exists, and we agree that we are justified in believing it exists because there is a document, but rather the document is effectively the corporation. The thing we as a society agree about is merely linguistic in nature: We agree on the definition of the terms, i.e., we have a common language.