PdfCommon.Initialize();

var doc = PdfDocument.Load("ct600-2008.pdf");

PdfPageCollection pageCollection = doc.Pages;
int count = pageCollection.Count;

PdfPage pdfPage = pageCollection.ElementAt(0);

PdfPageObjectsCollection pageObjects = pdfPage.PageObjects;
count = pageObjects.Count;

PdfTextObject txtObj = PdfTextObject.Create();
//Firstly you need to set the font to this text object
txtObj.Font = PdfFont.CreateStock(doc, FontStockNames.Arial);
//Next you need to set the text's color
txtObj.StrokeColor = Color.Black;
//After that you need to set the text's size
txtObj.Transform(50, 0, 0, 50, 0, 0);
//and then you can set the location
txtObj.Location = new PointF(50, 50);

//Now this line will be executed without any errors
txtObj.TextAscii = "Hello";
pageObjects.InsertObject(txtObj);