Scrivener allows you to split up a long text into smaller, more manageable chunks (chapters, scenes, main points - how large or small is up to you) and to edit them independently or together as one long document.
Its structural tools include a corkboard and an outliner for rearranging the constituent parts of your draft, so that you can plan your work in advance or get an overview and restructure it after the first pass. You can also refer to research documents (such as images and PDF files) by having them alongside your text as you work.
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