What case are "Beşinci sayfasını"?
It's accusative case since it's the direct object in this sentence.
that explains the -sı suffix but what about -nı? Why is that there?
I think raporun sayfası is the possessive (the report's page) + n (buffer) when accusative follows possessive + ı for the accusative.
Still confusing. Isn't it?
Yep! Need to just keep practicing it till it comes to you naturally. That's how kids learn a language, they do not have an inkling of tbe grammer