If you are not sure when you downloaded the document, probably the easiest method is simply to download it again and save it as any file name and to any folder you wish.  The latest updates are always maintained on the server.

An alternative method is to open the diagram PDF document and go to File > Properties. Under "Document Properties", see when the document was last modified as illustrated below. 

The modified date should be the same as the last recorded update available on the server for that document (see illustration below).  If the document's modified date is earlier than that on the server, download the document again to obtain the latest edition.