I did something like this myself. Take a large PDF, summarize each page. Make sure to have the titles of previous 3 pages, it helps with consistency and detecting transitions from one part to another. Then you take all page summaries in a list, and do another call to generate the table of contents. When you want to use it you add the TOC in the prompt and use a tool to retrieve sections on demand. This works better than embeddings which are blind to relations and larger context.

It was for a complex scenario of QA on long documents, like 200 page earning reports.