doc.ExportAsFixedFormat(OutputFileName=pdf_file,
ExportFormat=17, #17 = PDF output, 18=XPS output
OpenAfterExport=False,
OptimizeFor=0, #0=Print (higher res), 1=Screen (lower res)
CreateBookmarks=1, #0=No bookmarks, 1=Heading bookmarks only, 2=bookmarks match word bookmarks
DocStructureTags=True
);
As an alternative to the SaveAs function, you could also use ExportAsFixedFormat which gives you access to the PDF options dialog you would normally see in Word. With this you can specify bookmarks and other document properties.
The full list of function arguments is: 'OutputFileName', 'ExportFormat', 'OpenAfterExport', 'OptimizeFor', 'Range', 'From', 'To', 'Item', 'IncludeDocProps', 'KeepIRM', 'CreateBookmarks', 'DocStructureTags', 'BitmapMissingFonts', 'UseISO19005_1', 'FixedFormatExtClassPtr'
Source: https://stackoverflow.com/a/50516507/1667241