For HTML2PDF and XML2PDF solution check our PDFDoc Scout ActiveX library instead

using System;
using System.Collections.Generic;
using System.Text;
using Bytescout.PDF;
using System.Diagnostics;

namespace HelloWorld
{
class Program
{
static void Main(string[] args)
{
// Create main PDF Doc Engine
PDFDocEngine engine = new PDFDocEngine("", "");

// Add new document
Document document = engine.AddDocument("Input.pdf");

// get the page
Page page = document.GetPage((uint)(document.PageCount - 1));

// Create new drawing
Drawing drawing = page.AddDrawing();

// Add standard font
uint font = document.AddFontStandard(StandardFontType.Courier, FontEncodingType.WinAnsi);

// font size constant
const int fontSize = 50;

// Set Active Font
drawing.SetActiveFont(font, fontSize, false, false);

// get text width
double textWidth = drawing.GetTextWidth("Click me to open www.live.com");

// X position of the text to draw
const int textX = 100;
// Y position of the text to draw
const int textY = 350;

// Draw Text
drawing.PlaceText(textX, textY, 0, "Click me to open www.live.com");

// Closing drawing on the page
drawing.Close();

// create new action opening URL to live.com
ActionURL actionLink = new ActionURL(document, "http://www.live.com/", false);
// comments: use slashes "/" as a separator.
// to open external PDF document located in the root directory ( for example "c:" drive root folder) just add slash "/" as it means "root folder"
// to open external PDF in up-level folder use path like "../ExternalPDF.pdf"

// create a rectangle holding information about link position
Rectangle rectLink = new Rectangle();
rectLink.Left = textX;
rectLink.Right = textX + textWidth;
// IMPORTANT: Y coordinate scale for annotations, links, actions etc is INVERTED (due to PDF format nature) and calculating as 0 point at left-bottom and maximum at left-top
rectLink.Top = drawing.Height - textY; // so we calculate RectLink.top = height - offset from top
rectLink.Bottom = rectLink.Top-fontSize; // so we calculate rectLink.bottom as inverted

// set new link to the given rectanlge assigning an action opening external PDF
AnnotationLink Link = new AnnotationLink(page, rectLink, actionLink, HighlightingModeType.Invert, false /* set to true to show rectangle for the link in the document*/);

// Save document
document.Save("WithURLAddedOnPage.pdf");

// open generated PDF document in default PDF viewer installed in Windows
Process.Start("WithURLAddedOnPage.pdf");
}
}
}

Download example source code: bytescoutpdf_add_url_in_text_in_existing_pdf.zip (6 KB)

For HTML2PDF and XML2PDF solution check our PDFDoc Scout ActiveX library instead