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// For complete examples and data files, please go to https://github.com/groupdocs-watermark/GroupDocs.Watermark-for-.NET
var inputFolder = SourceFolderPath;
var outputFolder = SourceFolderPath + "/output";

var files = Directory.GetFiles(inputFolder);

var font = new Font("Arial", 8, FontStyle.Bold);
var watermark = new TextWatermark("CONFIDENTIAL", font);
watermark.HorizontalAlignment = HorizontalAlignment.Center;
watermark.VerticalAlignment = VerticalAlignment.Center;
watermark.RotateAngle = -45;
watermark.SizingType = SizingType.ScaleToParentDimensions;
watermark.ScaleFactor = 0.8;
watermark.Opacity = 0.5;
watermark.ForegroundColor = Color.Red;

foreach (var file in files)
{
    try
    {
        using (var doc = Document.Load(file))
        {
            doc.AddWatermark(watermark);
            doc.Save(Path.Combine(outputFolder, Path.GetFileName(file)));
        }
    }
    catch (UnsupportedFileTypeException)
    {
        Console.WriteLine("File format is not supported. File = {0}", Path.GetFileName(file));
    }
}