PageUp/PageDown does not work until after "Compare"

[reikred]

This is on linux, ubuntu 22.04.3, have not tried anywhere else:

diffpdf two files, say A and B. After the files open, PageUp/PageDown keys do nothing. After you hit the Compare button, then PageUp/PageDown styart working. In contrast, you do not have to hit the Compare button for the Previous/Next buttons to work.

More generally, I could not find any documentation of keyboard shortcuts. Not in manpage, not in README.md.