Nowadays it is getting more important to prevent crappy viewers from opening the document. This is bing a necessity when the document has intelligence which is not supported by the in-browser viewers or ( is a special case anyway because it destroys an intelligent PDF when it saves it). For this scenario there are ways to make sure that the downloaded document gets opened only in suitable viewers (which means Adobe viewers). For vanilla PDFs which have no intelligence and are very simplistic it is not really necessary anymore to have a . But as soon as it matters what opens the document it may be a good idea.