“A document usually adheres to some convention based on similar or previous documents or specified requirements.” https://whatis.techtarget.com/definition/document.
The topic of the discussion, as others have mentioned is: Time and context and which is it that makes a document? If the above quote holds any substantial truth (and I believe it does) then time and context would seem inextricably linked together in some way: “Time and context are not mutually exclusive” (Dickinson, 2011) https://www.oca.ac.uk/weareoca/photography/what-makes-a-document/#comment-4099
A document is something preserved, but based already in the context of history as a recording process that has a convention which allows it to be perceived as something worth retaining. The time factor will certainly change the way that the document is perceived. Jose makes this point. The context has identified the image as a document in relation to historical events. The time process, has a part to play in that with passing time, the document can be understood in a greater and clearer context of the civil war in Spain and all of the complexities that go with that. Either way, the photo still remains a document. What I mean by this is that a document is simply a form of information in one sense. This information may change over time as the context broadens (again tied in with time) but from the immediate development of the photo it is now already a document. All of the information that goes into securing a bigger context usually takes time to understand.