Edit the bookmark outline of a PDF - the navigation panel a reader shows down the side of the document. You can add entries, rename them, delete them, change which page they point to and nest them into levels. A merged document usually needs this: the bookmarks from each source file survive the merge, but as a flat run of unrelated entries rather than a structured contents list.
Bookmarks are navigation metadata shown in the reader sidebar. A contents page is printed content on a page. This tool edits the former, and does not change anything visible in the document body.
Yes - start from an empty outline and add entries pointing at the pages you want, which is how you give a plain scan usable navigation.
That feature reads the bookmark outline, so a document without one has no chapter boundaries to find. Adding bookmarks here first makes chapter splitting work.
Files are uploaded over an encrypted connection and processed on CoonTool's own server, not in your browser. The working copy is deleted automatically once the job is done, and in any case within 24 hours. Nothing is stored permanently and no account is required.