Hey there! I have a PocketBook Era, so hopefully I can help.
I use Calibre to send books to my device, and it works great for that. I plug the Era into my computer, Calibre sees it, tells me which books are on the device, and I can send books by clicking the "Send to Device" button. It is pretty seamless.
The Era shows up like a flash drive would with the OS, so you can just drag books onto it too.
I use KOReader on it, but I just played the stock reading software, and clicking the back button does not return to your previous position when clicking a link. However, when you click a link, a little box pops up in the bottom left corner with an arrow and the page number you were last on. If you tap the box, it will jump back to where you were.
And that is neat that you can use it to measure things like a scale. I have not tried that feature
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