You can export Outlines using the File menu on macOS and by using the context menu in the Documents view on all platforms.
PDF Doc
This exports a PDF file by treating the Outline as a document. See The Notes Field.
PDF List
This exports a PDF file as a traditional outilne.
Markdown Doc
This exports a Markdown file by treating the Outline as a document. See The Notes Field.
Markdown List
This exports a Markdown file as a traditional outline using Markdown’s list syntax.
OPML
This exports an OPML file as a traditional outline. It uses some extensions to the OPML spec that are common among outliners. It also uses Markdown for formatting inside text fields.