Word's navigation pane ignores headings sitting inside table cells. Interestingly, this issue extends to the file when it is saved in PDF format. In Acrobat the 'Navigation Pane' equivalent is called 'Bookmarks' and the headings which were in Word tables will be missing there too. You'll have to manually add those bookmarks in a PDF editor if you need them there.

However, a table of contents WILL include the headings which happen to be sitting in tables - that is the only alternative that I know of.

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Andrew Lockton
Chrysalis Design, Melbourne Australia