I'd upgrade to a 4K TV if I had a viewer for it.
I'd pony(!) up $200 for an app that did that. I don't know if other people would, but I would.
I find it strange that Roku comes with an app to play movie files, display jpgs, play song files, but it won't display a pdf. I've searched Roku for such an app, and googled for it, but came up empty.