Open the file in the Designer not in a text editor. Yes, it's an XML file which describes the UI but it is not human-readable (almost ). Therefore
(1) If you have Designer installed, run the Designer, select "Open", select your ui file.
(2) Otherwise, open your project in the Creator, open "forms", select your ui.file.

You will see your UI, check attributes of MainWindow and see, whwther it has the scrollbar (I am not sure it can, You will need QScrollArea for it).