I'm back at Dropbox's PDF viewer. Notice the grey bar at the bottom that contains some functionality related to the document: navigation, fullscreen toggle, print button etc.
Of course it's animated. Of course it disappears and appears. But notice this: it appears on every single action that a user is performing. You scrolled the document? It pops up. You moved the mouse? It pops up. The only way for it to remain hidden is for to you stare at the screen and do one thing only: read every word from top to bottom without even adjusting the document's scroll position. I repeat: do not touch that mouse/trackpad.
Because, obviously, there's just not enough space in that entire page to place this bar. Not even in the 2000 or so empty pixels between the document name and the Download button.