The user is asking for a feature that's "extra quality." So, I should think about what features make a PDF reader or viewer better. Let's brainstorm some ideas. Usability features: maybe OCR for searchable text, adjustable fonts, dark mode. Educational tools: annotations, highlighting, bookmarking, note-taking. Maybe language support since Mir books are Russian, so translation or dual-language options. Accessibility: screen readers, voice reading. Collaboration features like sharing notes. Organization tools like tagging or categorization.