Add watermarks to PDF documents

using GemBox.Pdf;
using GemBox.Pdf.Content;
using System;

class Program
{
    static void Main()
    {
        // If using the Professional version, put your serial key below.
        ComponentInfo.SetLicense("FREE-LIMITED-KEY");

        using (var document = PdfDocument.Load("%InputFileName%"))
        {
            using (var formattedText = new PdfFormattedText())
            {
                // Format the watermark text.
                formattedText.FontFamily = new PdfFontFamily("Calibri");
                formattedText.Color = PdfColor.FromGray(0.75);
                formattedText.Opacity = 0.5;

                // Set the watermark text.
                formattedText.Append("CONFIDENTIAL");

                foreach (var page in document.Pages)
                {
                    // Make sure the watermark is correctly transformed even if
                    // the page has a custom crop box origin, is rotated, or has custom units.
                    var transform = page.Transform;
                    transform.Invert();

                    // Center the watermark on the page.
                    var pageSize = page.Size;
                    transform.Translate((pageSize.Width - formattedText.Width) / 2, 
                        (pageSize.Height - formattedText.Height) / 2);

                    // Rotate the watermark so it goes from the bottom-left to the top-right of the page.
                    var angle = Math.Atan2(pageSize.Height, pageSize.Width) * 180 / Math.PI;
                    transform.Rotate(angle, formattedText.Width / 2, formattedText.Height / 2);

                    // Calculate the bounds of the rotated watermark.
                    var watermarkBounds = new PdfQuad(new PdfPoint(0, 0),
                        new PdfPoint(formattedText.Width, 0),
                        new PdfPoint(formattedText.Width, formattedText.Height),
                        new PdfPoint(0, formattedText.Height));
                    transform.Transform(ref watermarkBounds);

                    // Calculate the scaling factor so that rotated watermark fits the page.
                    var cropBox = page.CropBox;
                    var scale = Math.Min(cropBox.Width / (watermarkBounds.Right - watermarkBounds.Left),
                        cropBox.Height / (watermarkBounds.Top - watermarkBounds.Bottom));

                    // Scale the watermark so that it fits the page.
                    transform.Scale(scale, scale, formattedText.Width / 2, formattedText.Height / 2);

                    // Draw the centered, rotated, and scaled watermark.
                    page.Content.DrawText(formattedText, transform);
                }
            }

            document.Save("Watermarks.%OutputFileType%");
        }
    }
}