I have been thinking about this, but for a while now, I have settled on using ChatGPT's GPT-4v's multimodal capability to generate a text file containing the ti

I have been thinking about this, but for a while now, I have settled on using ChatGPT's GPT-4v's multimodal capability to generate a text file containing the titles and pages based on screenshots of the TOC. After that, I used a pikepdf-based Python script to bake the TOC into the PDF I had. The upside, compared to Krasjet's approach, is that this works not only for text-based PDFs but also for scanned PDFs, even old scanned journal papers. The downside is that, before baking the TOCs, you need to make adjustments to the PDF as sometimes the empty pages are not included. You also need to calculate the offset for the prologs, cover, etc. I have a script for this kind of adjustment, but there always is manual intervention involved.