Why do I see someone else's annotations and highlighting on my pdf?

I'm in grad school and we have a textbook; last week I made some highlights and comments on the copy I downloaded and this week I see mine but also someone else's... I need my own copy without someone else's notes on it. But also worried my own comments and notes are public(??) which I do not want. Something to do with signatures?? Can anyone help please? Why is this even happening?!