PDFsharp & MigraDoc Foundation

const string ImagePath = @"C:\pexels-eberhardgross-443446.jpg";
const string PdfPath = @"C:\foo.pdf";

using (var imageStream = File.OpenRead(ImagePath))
{
    var image = new MagickImage(imageStream);
    image.Resize(new Percentage(20));
    image.Quality = 50;

    using (var resizedImageStream = new MemoryStream())
    {
        image.Write(resizedImageStream);
        resizedImageStream.Position = 0;

        using (var document = new PdfDocument())
        {
            PdfPage page = document.AddPage();
            page.Size = PdfSharpCore.PageSize.A4;
            using (XImage img = XImage.FromStream(() => resizedImageStream))
            {
                XGraphics gfx = XGraphics.FromPdfPage(page);
                var pageWidth = gfx.PageSize.Width * 0.95;
                var pageHeight = gfx.PageSize.Height * 0.95;

                // compute image width / height maintaining image aspect ratio
                var width = pageWidth;
                var height = (((double)width / (double)img.PixelWidth) * img.PixelHeight);
                if (height > pageHeight)
                {
                    height = pageHeight;
                    width = (((double)height / (double)img.PixelHeight) * img.PixelWidth);
                }

                gfx.DrawImage(img, (gfx.PageSize.Width - width) / 2, (gfx.PageSize.Height - height) / 2, width, height);
            }
            using (var pdfStream = File.OpenWrite(PdfPath))
            {
                document.Save(pdfStream);
            }
        }
    }
}