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// For complete examples and data files, please go to https://github.com/groupdocs-watermark/GroupDocs.Watermark-for-.NET
using (WordsDocument doc = Document.Load<WordsDocument>(Utilities.MapSourceFilePath (FilePath)))
{
    using (ImageWatermark watermark = new ImageWatermark(@"D:\large.png"))
    {
        // Add watermark to all headers of the first section
        doc.Sections[0].AddWatermark(watermark);
    }

    // Link all other headers&footers to corresponding headers&footers of the first section
    for (int i = 1; i < doc.Sections.Count; i++)
    {
        doc.Sections[i].HeadersFooters.LinkToPrevious(true);
    }

    doc.Save();
}