Quill OS is an open-source, fully-functional standalone OS for Rakuten Kobo's eReaders.
Here are some of Quill OS' features:
Fully integrated KoBox X11 subsystem
ePUB, PDF, picture and plain text display support
Versatile configuration options for reading
muPDF rendering engine for ePUBs and PDFs
Wi-Fi support and web browser
Encrypted storage with EncFS
Fast dictionary & local storage search
Dark mode
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