If you're on Windows simply hover your cursor over document you want to scroll through, while keeping focus on other window and scroll the scroll wheel on mouse.

You can keep using keyboard for input/scrolling of active window, as you won't lose focus on your current active window by scrolling with mouse.

If you use touch pad instead of mouse on your laptop probably common scroll gesture will work.

Info about pointers by Microsoft

More about gestures for Windows 10

Probably it's going to work on others OSs too.

Happy coding ;)