How are you doing the conversion to PDF - via Word's Save As PDF, a third party Save to PDF tool, or with a PDF 'printer'?
Have you tried repairing the Office installation (via Word Options|Resources|Diagnose)? Is your Office 2007 installation fully updated? See also:
https://support.microsoft.com/en-us/kb/969691
It would be unwise to unlink your Table of Contents (e.g. via Ctrl-Shift-F9) as has been suggested; while that might work around the problem you're now having, it also means the Table of Contents won't ever update to accommodate any existing or further edits
or even the repagination that can occur just by opening the document on a computer using a different active printer than when it was last updated. If you need a short-term workaround, update then lock the Table of Contents before printing, via F9 then Ctrl-F11,
then unlock it again after printing, via Ctrl-Shift-F11.