Is there an SDK or open source libraries that you've developed for the device that developers could leverage? It seems that you will support the android app store, but will there be a separate Sol:OS store for more tailored apps?

Why not use existing libraries, like Poppler?

Agreed. This screams scope creep.

"We've built our own renderer" would make sense to me, perhaps specific handling is needed for the device's display.

However rolling a parser does not without further context, it screams: "I want to encounter some dumb non-specification stuff and crash for no reason"

Why PDF first? Isn't reading Ebooks (in ebook format) the main use case for such devices?

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.

Is the kindle app not supported?

Is Kindle on Android? Yes.

These are the apps they are specifically building. But all Android apps are availabe for it as well.

I mean, did they specifically check that the Kindle app works well on their OS.