However browsing the filesystem of the device I found that the highlights, notes and snapshots can be easily extracted.
If you use Windows, connect your PockeBook to the PC. In the file explorer on the top menu bar (hit Alt to show it), click on Tools/Folder settings. You have to set 'Don't hide system files', also 'Show hidden files'. Now you can browse the system files.
The highlights and comments for each book can be found in HTML format under the following path:
Btw this is the InkPad 840