Normally screen's DPI (KDE System Settings → Application Appearance → Font Settings → Force Font DPI) is set to about 100; I have it set to 250 since I have a very high DPI screen (Lenovo Yoga 2 Pro).

The font in an ePub book seems to depend on the desktop DPI. It shouldn't, since the font size relative to a page should be fixed and the page size scaled to fit the screen. What this means is that I see very few words per page (around 5 per line, 12 lines per page in one sample), which is nearly unusable. I have checked that setting the DPI lower increases words per page.

I'm guessing there is meant to be a way to control the "font size relative to a page", but desktop DPI shouldn't be that control. In my case I can have two large pages side-by-side each over 1000 pixels wide but containing very few words or I can shrink the pages to get a reasonable font size but leave half the screen unused (and very short lines of text).

Reproducible: Always