Impress read-only mode can't navigate with Page Up Down keys

When opening document in Read-Only mode it seems that some features are disabled.

The Page Up / Page Down keys are not working as expected and slides are not advancing/reversing.

Home and End keys do work.

I have the same issue on LO 5.4.5.1; the solution seems to be to go into Edit mode. Kind of baffling, though!