using DevExpress.Drawing;
using DevExpress.Pdf;

Stream ApplyWatermark(Stream documentStream, string text, bool rotate) {
    using var processor = new PdfDocumentProcessor();

    processor.LoadDocument(documentStream);

    using var brush = new DXSolidBrush(System.Drawing.Color.FromArgb(50, System.Drawing.Color.LightSkyBlue));

    var fontName = "Arial Black";
    var fontSize = 12;
    var font = new DXFont(fontName, fontSize);

    var stringFormat = PdfStringFormat.GenericTypographic;
    stringFormat.Alignment = PdfStringAlignment.Center;
    stringFormat.LineAlignment = PdfStringAlignment.Center;

    foreach(var page in processor.Document.Pages) {
        using var graphics = processor.CreateGraphicsPageSystem();

        var watermarkSize = page.CropBox.Width * 0.75;

        var stringSize = graphics.MeasureString(text, font);
        float scale = (float)(watermarkSize / (double)stringSize.Width);

        graphics.TranslateTransform((float)(page.CropBox.Width * 0.5), (float)(page.CropBox.Height * 0.5));
        if(rotate)
            graphics.RotateTransform(-45f);
        graphics.TranslateTransform(
            (float)(-stringSize.Width * scale * 0.5),
            (float)(-stringSize.Height * scale * 0.5));

        var actualFont = new DXFont(fontName, fontSize * scale);
        var rect = new System.Drawing.RectangleF(0, 0, stringSize.Width * scale, stringSize.Height * scale);
        graphics.DrawString(text, actualFont, brush, rect, stringFormat);

        graphics.AddToPageForeground(page);
    }

    var outputStream = new MemoryStream();
    processor.SaveDocument(outputStream);

    return outputStream;
}