I find seeing were I am in a document difficult in the pages thumbnails.
As I scroll through a long document there's no indication of the page I'm looking at until I click an element on the screen page. And then the "indicator" is just the subtle highlighting of the text and a colour overlay on the thumbnail.
Feels like a legacy thing. If it was created new today we'd use something far more obvious.
I'd like to see a rectangle floating over the pages that matches the frame of the main document window (for example).
Or at least a selected page needs to stand out far more than it does. Right now it takes some serious visual searching to see what is selected. Make it larger, frame it, make it glow... something that shows in a quick scan.

Thanks!
N.