You can when you create PDF files, to add a "bookmark" electronically.
"Bookmark" say is not intended to pinch far as can be seen when viewing or reading, is close to Table of Contents here. Once you have added a "bookmark" when creating PDF files, the viewer simply click on the section you want to read in the "bookmarks", the page will be displayed.
It is not one that must be added, but many PDF files that have been published without the "bookmark" Rather, if you want to create a PDF file large number, especially a "bookmark" even bother little viewer is very useful if you add.