You would like to use Bookmarks to navigate a PDF, but the Bookmarks panel is not available. Why would that panel be unavailable?

Here is the answer to this question: You would like to use Bookmarks to navigate a PDF, but the Bookmarks panel is not available. Why would that panel be unavailable?

The PDF does not contain any articles.
You are viewing the PDF in a web browser.
The file creator did not give the article a title.
You are viewing the PDF in Adobe Reader.