This article shows how to add text to a pdf.

To use these methods you need a Premium license

            StorageFolder storageFolder = ApplicationData.Current.LocalFolder;
            StorageFile newFile = await storageFolder.CreateFileAsync("new.pdf", CreationCollisionOption.ReplaceExisting);
            IRandomAccessStream stream = await newFile.OpenAsync(FileAccessMode.ReadWrite);

            PDFDoc newDoc = new PDFDoc();
            if (newDoc.Create(stream) == PDF_ERROR.err_ok)
            {
                float width = 297f, height = 210f;
                PDFPage page = newDoc.NewPage(0, width, height);

                PDFDocFont dfont = newDoc.NewFontCID("Arimo", 1 | 8);//bold and embed in horizontal writing
                if (dfont != null)
                {
                    PDFPageFont pageFont = page.AddResFont(dfont);
                    if (pageFont != null)
                    {
                        PDFPageContent pageContent = new PDFPageContent();                                        

                        int x = 100, y = 100;
                        float fontSize = 20, charSpace = 0, wordSpace = 0.3f;
                        String text = "RadaeePDF\rTest!";

                        //add text highlight 
                        pageContent.GSSave();
                        PDFDocGState dgs = newDoc.NewGState();
                        dgs.SetFillAlpha(100); //alpha
                        dgs.SetBlendMode(2); //Multipy
                        PDFPageGState rgs = page.AddResGState(dgs);
                        pageContent.GSSet(rgs);
                        PDFPoint textSize = pageContent.GetTextSize(text, pageFont, fontSize, fontSize, charSpace, wordSpace);
                        float textHalfHeight = textSize.y / 2;
                        PDFPath path = new PDFPath();
                        path.MoveTo(x, y - textHalfHeight);
                        path.LineTo(x + textSize.x, y - textHalfHeight);
                        path.LineTo(x + textSize.x, y + textHalfHeight);
                        path.LineTo(x, y + textHalfHeight);
                        path.Close();
                        pageContent.SetFillColor(0x0000FF);
                        pageContent.FillPath(path, true);
                        pageContent.GSRestore();

                        //add text
                        pageContent.GSSave();
                        PDFMatrix matrix = new PDFMatrix(1, 1, x, y);
                        pageContent.GSSetMatrix(matrix);

                        pageContent.TextBegin();
                        pageContent.TextSetFont(pageFont, fontSize); //set font and size
                        pageContent.SetFillColor(0xff0000);
                        pageContent.SetStrokeColor(0x10);
                        pageContent.TextSetCharSpace(charSpace);
                        pageContent.TextSetWordSpace(wordSpace);
                        pageContent.TextSetLeading(20);
                        pageContent.TextSetRenderMode(2);//fill and stroke
                        pageContent.TextSetHScale(100);//set horizontal scale
                        pageContent.DrawText(text);                      
                        pageContent.TextEnd();
                        pageContent.GSRestore();

                        page.AddContent(pageContent, true);
                        page.Close();
                    }
                }
                newDoc.Save();
                newDoc.Close();
            }