Make it possible to create dictionary entries while in the reader, and have the option to have said entries be limited to that specific file/book.
I'm currently reading a book that has a lot of pseudo-English names (made up by the writer) which are transcripted into Chinese characters. And while I have memorized the transcription table, it is sometimes a bit difficult to keep track of characters/place names, or even specific items within a story. Some light novels have a thousand+ chapters, and a light novel like 《武炼巅峰》has a story that is almost 18 million characters long. My story isn't even a fraction of that, and I'm starting to lose track. I would search the documents for mentions of said person/place, but due to my novel having a few thousand pages it also crashes every time I do a search. So, having a kind of system where you can make dictionary entries specific to a document would be really helpful.