In the accessibility world, PDF gets a decidedly mixed review. On the one hand, PDF is an extraordinarily flexible vehicle capable of providing accessible structure to documents from almost any source. On the other hand, this same flexibility makes quality-control a challenge, and understandings vary about what is and is not necessary in order to consider a given file "accessible and compliant with Section 508".
For these reasons, it is always vastly preferable that the structure information necessary for PDF accessibility be provided by the authoring software when the PDF is created. Any other solution involves PDF editing expertise that very understandably confuses and frustrates many users; all the more so because it should be unnecessary.
The solution? There is no good solution -- for now. At the present time, Word can't make a Word file that complies with Section 508, paragraphs (g) and (h), so don't expect it to make a compliant PDF either. Were you hoping that Open Office might do better? Me too. Think again; OO 3.1 slavishly follows Word, and includes the exact same limitations.
Those with Adobe Acrobat Professional may correct their table structures after PDF creation. Those distributing Word files are stuck.
Manufacturers of authoring software should focus on the essentials in their own applications -- and that certainly includes getting table structure right! The coming Section 508 "refresh" is considerably more demanding than the existing regulations. Better authoring tools are in everyone's interest, and soon.
OK, so you want to stamp your document. Maybe you need to give reviewers some advice about the document's status or sensitivity. This tip from author Ted Padova demonstrates how to add stamps with the Stamp Tool along with related comments.