212 PPI could be revolutionary in viewing non-text documents on an e-ink screen. The current Kindle is basically unusable for PDFs unless you have a DX. It might still be too low of a DPI, but it has to be a major improvement and right now most PDFs are just BARELY unreadable.
Back in 1990 I saw a prototype IBM display that was 230PPI which really impressed me. The person doing the demo said that eyestrain drops dramatically once you're eye can more easily infer the lines rather than connecting the dots on 'large' pixels. Their claim was that 200 PPI was the reading threshold where eyestrain due to pixelation became 'negligible'.
At one time I had a link to some studies but now I only find later papers, sigh. Searching for 'eye fatigue ppi' gets you some decent hits.
On the contrary, I find reading PDFs with reasonable margins (e.g. the kind you find in computer science papers) on my 3rd gen Kindle very easy, so long as I flip the Kindle into landscape mode (for one-column documents) or zoom in a little bit (for two-column documents).
