If you say something like "I read an article about grammar, which I didn't understand", it's ambiguous whether "which" refers to "article" or "grammar". In this specific example it's pretty clear from context, but it could be a bit confusing with a different object ("I read an article about the new theory, which I didn't understand.")

I'd just rephrase the whole sentence to avoid ambiguity if it were a problem because saying "which article" doesn't quite feel right in this context—a bit too formal and old-fashioned?—but this use of "which" seems like a reasonable way to disambiguate if it fits in context.