Unlike the very, very basic User Guide PDF, HP has a site that provides more information about using the Slate 8 Pro.
This section is about the HP File Manager and contains this spoilsport — and rather WTF — notice:
HP File Manager displays in landscape mode only; the screen does not rotate to portrait mode, even with screen rotation enabled. Hold the tablet in landscape orientation while using HP File Manager.
Seriously, HP? You couldn’t be bothered to also give it Portrait Mode — the way most people will hold the tablet?
It will be a measure of how serious HP is about the Slate 8 Pro to see how soon they kick out an update to File Manager to give it Portrait Mode too. This is 2013 and Landscape-only is just inexcusable.
Same-day update: Someone at HP needs a slap. All of the words next to the icons — Refresh, etc — are superfluous. These are all Android conventions and people would understand the icons alone. Also, advanced users are the ones likely to use the File Manager, not novices, so the words are doubly superfluous. Delete the words and Portrait Mode is easily created. Also, shrink the icons for My Storage and SD Card. Those are just crazy big.