NOTE: You can’t open these TOC settings if you’ve inserted a default (Microsoft-supplied) TOC from the References tab-you must have inserted a custom TOC.
I checked their TOC settings and there I found the reason and the solution.
Other elements such as cross-references to sections, appendices, tables, and figures all worked fine in the PDF, but not the TOC entries. They had used standard Heading styles, so there was no reason why they shouldn’t be linked. because the standard settings for clickable links in a PDF are to include the usual Heading styles from Word. One of my clients had a situation where the table of contents (TOC) in their Word document did not become a clickable (linked) TOC in the PDF they created from it.
