Once you're approaching regular paper size, PDF is king.

There are just so, so many documents out there which were laid out with A4 paper in mind, which can't really be converted to any other format with reasonable effort. Just think of every research paper published ever.

People who are primarily reading books aren't really in the market for this kind of device. You can read books just fine on any old kindle and it's far more comfortable on a smaller, lighter device.

Likely to focus on use cases for their display tech. Ebooks are already solved and faster refresh won't meaningfully improve the experience over other devices.