One comment here (not specific to Onyx M92 but specific to 10" ereaders): since I am reading lots of PDFs these days I just that the full size PDF (either letter or A4) present both a challage and an advantage.
The advantage is that in case of diagrams or anything that requires you to go back and forth on the same text in order to understand it, in these situations a bigger paper format is an advantage
The disadvantage is built in within the very advantage indicated above: a bigger page makes the text reading more difficult since your eye sight has to go back and forth to follow the text line.
If you can take a little bit smaller font then the resizing done by the rendering process of any 10" display becomes an advantage because this makes the lines shorter
I believe that the only reason why the technical manuals do not use smaller paper format (where they can be) is because that format does not exist. That is an intresting question. What factors led to the standard sizes we all use today.
We now what the modern roads have to do with roman chariots :-)