c# iText7 如何设置段落左上角的位置

问：

我不想在pdf文件中写一个段落，以便段落的左上角正好位于页面100,100的左上角。

var targetPdfPath = "100x100.pdf";
using (PdfDocument pdfDoc = new PdfDocument(new PdfWriter(targetPdfPath)))
{
    Document document = new Document(pdfDoc);
    var page = pdfDoc.AddNewPage(PageSize.A4);
    Paragraph paragraph = new Paragraph("Must be at (100,100)");
    PdfFont FontHELVETICA = PdfFontFactory.CreateFont(StandardFonts.HELVETICA);
    paragraph.SetFontSize(20f);
    var pageHeight = PageSize.A4.GetHeight();
    PdfCanvas pdfCanvas = new PdfCanvas(page);
    var rectHeight = 200f;
    var rectWidth = 200f;
    pdfCanvas.SetLineWidth(0.5f);
    pdfCanvas.Rectangle(100, pageHeight - 100 - rectHeight, 200, 200);//It is easy to place the rectangle in the position of (100,100)pt
    pdfCanvas.Stroke();

    Canvas canvas = new Canvas(pdfCanvas, PageSize.A4);
    var paragraphHeight = 0f;//How to get it??
    paragraph.SetFixedPosition(100, pageHeight - 100f - paragraphHeight, 200);//It is difficult to place paragraphs in the position of (100,100)pt
    canvas.Add(paragraph);
}

无论我测试多少，我都无法获得正确的结果。我该怎么办？100x100.pdf