A book that exists off the screen
A title page, the cover, chapter openings and proper page breaks. Print it, or save it as a PDF.
The end of a story should be a thing you can put on a shelf.
What comes out
A title page with the cover on it, then each chapter starting on a fresh page with its own heading. Paragraphs do not break badly across pages, headings are never left stranded at the bottom of one, and the margins are set for something that might get bound.
Print or save
It opens your browser's own print dialog, which means you can send it to a printer or choose "Save as PDF" and keep it. That is deliberate: your browser does far better typography than anything this site could do on its own, it knows about your paper size, and the file it produces has real text in it rather than a picture of text.
Any story, at any length
Three chapters or thirty. There is nothing to export and nothing to wait for: the whole book is already a page, and printing it is one click from the story.