In Linux KDE this function is built-in: Right click on any Window title > Advanced > Keep above others
In Windows, this feature does not come shipped from factory. You need a 3rd party tool. This small program is useful.
Download, extract the executable and double-click; it'll now be visible in the system tray as an icon. Go to your window you want to make visible and use key combo Ctr+Space to enable/disable that widow to be always on top.
Together with WizMouse, this option is quite useful and will increase your productivity working with multiple documents at the same time. Viewing and comparing two large documents or spreadsheets becomes much efficient.
More details on the reference webpages below
____Reference Web Links______________________________________________
Always on Top: Keep Any Window Visible Always