var currentDir = Directory.GetCurrentDirectory();
var fileLocation = currentDir + @"\Data\Resources\sample_fillable_pdf.pdf";
var document = PdfReader.Open(fileLocation, PdfDocumentOpenMode.Modify);

string qrString = "encode-me";
CodeDataMatrix qrCode = new CodeDataMatrix(qrString, 26, 26);
XGraphics graphicsOnFirstPage = XGraphics.FromPdfPage(document.Pages[0]);
graphicsOnFirstPage.DrawMatrixCode(qrCode, new(50, 500));

Message: 
System.InvalidOperationException : 'XImage' must not be null here.

  Stack Trace: 
NRT.ThrowOnNull[TResult](String message)
MatrixCode.get_MatrixImage()
CodeDataMatrix.Render(XGraphics gfx, XBrush brush, XPoint position)
XGraphics.DrawMatrixCode(MatrixCode matrixcode, XPoint position)
PdfSampleReportManagerTests.TestFillPdfForm() line 38
RuntimeMethodHandle.InvokeMethod(Object target, Void** arguments, Signature sig, Boolean isConstructor)
MethodInvoker.Invoke(Object obj, IntPtr* args, BindingFlags invokeAttr)

using PdfSharp.Drawing;
using ZXing;
using ZXing.Common;

namespace OurApp.Utilities.Pdf;

/// <summary> Utilities for working with QR codes </summary>
public static class QrCodeHelper
{
    /// <summary> Create a BitMatrix from the given string.
    /// Caller must ensure that size (length of QR code square side in pixels) can handle the content to encode,
    /// otherwise be prepared to handle <see cref="WriterException"/>
    /// </summary>
    public static BitMatrix Encode(string qrCodeContent, int size = 50)
    {
        var qrCodeWriter = new BarcodeWriterGeneric()
        {
            Format = BarcodeFormat.QR_CODE,
            Options = new EncodingOptions { Width = size, Height = size, },
        };

        BitMatrix bitMatrix = qrCodeWriter.Encode(qrCodeContent);
        return bitMatrix;
    }

    /// <summary>
    /// A replacement for XGraphics.DrawMatrixCode() until implementation is finished
    /// https://github.com/ststeiger/PdfSharpCore/issues/361,
    /// https://forum.pdfsharp.net/viewtopic.php?f=2&t=4493
    /// </summary>
    public static void DrawQrCode(this XGraphics graphics, XPoint upperLeftCorner, BitMatrix bitMatrix, int pixelSize = 3)
    {
        for (int x = 0; x < bitMatrix.Width; x++)
        {
            for (int y = 0; y < bitMatrix.Height; y++)
            {
                var pixelLocation = new XPoint(upperLeftCorner.X + (x * pixelSize), upperLeftCorner.Y + (y * pixelSize));
                var rectangle = new XRect(pixelLocation, new XSize(pixelSize, pixelSize));
                var color = bitMatrix[x, y] ? XBrushes.Black : XBrushes.White;

                graphics.DrawRectangle(color, rectangle);
            }
        }
    }
}