Add a watermark to a PDF in C#

Since 1.1View as Markdown

Write a large, rotated, semi transparent word across every page.

Short answer

Draw on each page with CreateGraphics, rotate the canvas about the page center with RotateAtTransform, and draw the text with a color that has an alpha below 1. Positive angles turn clockwise, so -45 runs the text up from bottom left to top right.

#:package PdfNetStack@1.2.0
using PdfNetStack;
using PdfNetStack.Drawing;
using PdfNetStack.Fonts;

PdfDocument doc = PdfDocument.Open("contract.pdf");
PdfFont font = PdfStandardFont.HelveticaBold;
double size = 110;
double width = font.MeasureString("DRAFT", size);
foreach (PdfPage page in doc.Pages)
{
    using PdfGraphics g = page.CreateGraphics();
    g.RotateAtTransform(-45, page.Width / 2, page.Height / 2);   // positive angles turn clockwise
    g.DrawString("DRAFT", font, size, PdfColor.FromHex("#DC2626").WithAlpha(0.25),
        (page.Width - width) / 2, page.Height / 2 + size * 0.35);
}
doc.Save("contract-draft.pdf");

The first page of the contract with a red DRAFT watermark across it

Variations

page.CreateGraphics(PdfGraphicsLayer.Background) draws underneath the existing content, so the text of the page stays on top of the watermark.
For an image or a logo, use DrawImage or DrawSvg the same way. To stamp another PDF page, such as a letterhead, use PdfForm.FromPage and DrawForm, see Images.