...
pdfViewer1.DocumentLoaded += PdfViewer1_DocumentLoaded;
...


private void PdfViewer1_DocumentLoaded(object sender, EventArgs e)
{
    pdfViewer1.Document.Pages.PageLoaded += Pages_PageLoaded;
}

private void Pages_PageLoaded(object sender, Patagames.Pdf.Net.EventArguments.PdfPageEventArgs e)
{
    //Firstly you need to insert image as a background for your text

    //Create a PdfBitmap
    PdfBitmap bmp = new PdfBitmap(200, 100, true);
    bmp.FillRect(0, 0, 200, 100, Color.Red);
    
​    //Create image object and set previously created bitmap
    var img = PdfImageObject.Create(e.Page.Document);
    img.SetBitmap(bmp);

    //Move image object to 0,0 and set it's size to 1px;
    img.SetMatrix(1, 0, 0, 1, 0, 0);

    //Scale image object to it's actual width and heihgt
    img.Transform(bmp.Width, 0, 0, bmp.Height, 0, 0);
    
​    //Insert image object into page
    e.Page.PageObjects.InsertObject(img);

   
​    //Then you can insert your text
    PdfTextObject txtObj = PdfTextObject.Create();
    txtObj.Font = PdfFont.CreateStock(pdfViewer1.Document, FontStockNames.Arial);
    txtObj.FillColor = Color.Green;
    txtObj.Transform(50, 0, 0, 50, 0, 0);
    txtObj.Location = new PointF(50, 50);
    txtObj.TextAscii = "Hello";
    e.Page.PageObjects.InsertObject(txtObj);

    //redraw PdfViewer control
    pdfViewer1.UpdateLayout();
}