Never been to that specific mill but old paper mills in general have a really strange atmosphere, something about the combination of industrial decay and water proximity I think. Water seems to amplify whatever's going on in a place, always has in my experience.
What exactly did you feel? I ask because there's a difference between the usual "uneasy" feeling people get in derelict buildings (which is mostly just your brain reacting to the environment) and something more specific like directional pressure or the sense of being watched from above rather than around you. The latter two are things I've documented near what I'd loosely call interdimensional activity.
Not saying it's definitely that, just want to narrow it down before jumping to conclusions. Anyone else local to the area who's visited? Would be good to get a few accounts and see if theres any consistency to what people experienced.