Hidden security token in word and/or pdf files?

I want to know if it is possible to add some hidden data prog开发者_JAVA百科rammatically to a word and/or a pdf file.

Actually I am trying to add some security token to a file so it needs to be hidden inside.

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Not that reliable, we did add white text on white background and/or text with a tiny font size right inside a Word document. Good for short texts, in our special case.

You could use Aspose.Words or Aspose.Pdf (both commercial) to manipulate Word documents or PDF documents.