After editing a document (e.g. writing marked-up text), click the 'save' or the 'done' button to save your draft as preview version of your document.

The save button does nothing else, while the 'done' button will also leave edit_mode and bring the document in review_mode.

Even if you do not press the 'save' button, your changes are saved automatically, but in the draft version of your document. You can discard the draft version of your document, but you can not discard the preview version of your document.