It only lists text that's actually styled as a heading
The Navigation Pane builds its outline from text formatted with heading styles like "Heading 1" or "Heading 2" -- plain text that's just made bold or bigger by hand won't show up as an entry, even if it visually looks like a heading. If a document's outline pane looks empty or incomplete, applying proper heading styles to your section titles usually fixes it.
It doubles as a thumbnail browser, not just an outline
Switching the tab at the top of the pane from headings to "Pages" replaces the heading list with a small thumbnail image of every page, letting you scan a document visually and jump to a page by its layout or a photo on it rather than by title. That's handy in a document where you remember roughly what a page looked like but not what section it was under.
Frequently Asked Questions
No headings show up in the Navigation Pane at all.
The pane only recognizes text formatted with heading styles like "Heading 1" or "Heading 2." If text is only made bold or enlarged manually, apply an actual heading style to it so it appears in the list.
Can I browse page thumbnails instead of a list of headings?
Yes. Selecting "Pages" from the tab at the top of the pane replaces the heading list with small preview images of each page, so you can find a page visually.