OK, I have tested the latest version of KOReader, and the DRCOUNTMAX setting (
https://github.com/koreader/koreader...hange-defaults) affects both text and image documents.
So thanks for suggesting that! I hoped for a hack for the stock PDF viewer, but using KOReader is fine for me too.
However, I now have an issue with the nonexistent left margin while viewing this PDF, I will post in KOReader's forum.