I don't know for sure in the exact context you're referring to but assuming there is some sort of remotely methodical sign convention applied to this then I'd say it must decrease, since -5 is less than -3 say for example. If it doesn't follow this then by saying increasing pitch angle it is still fairly ambiguous, whereas using a proper sign convention removes the ambiguity and can only mean pitching down, be it from +10 to +5 or -1 to -3, it doesn't matter.
Hope this helps a bit, and I'll repeat that I don't know for sure in this specific application, so if someone does know then I'm open to correction but this seems by far the most sensible way to tackle this problem.