To me it's koreader (or co-reader if you find that clearer).
At first it was written KoReader, which pronunciation-wise seems clearer to me, but then it was changed to KOReader because apparently KoReader suggested a pronunciation like Korea to some. Which is a "problem" if you say kuh-ree-uh but not if you say koh-ree-uh.
I find kuh-READer a bit implausible as a reading for koreader myself, but perhaps as a native speaker of Dutch I have a stronger initial stress bias than English speakers. Or maybe it's non-native English speakers of non-Germanic origin who actually have the stress issue.
For historical context:
https://github.com/koreader/koreader/pull/25
https://github.com/koreader/kindlepdfviewer/issues/782