If you create a custom viewer for PDF documents, then you could try Apitron PDF Rasterizer library that has unique features designed specifically to meet the needs of custom viewer writers. In addition to its core rendering functionality, it offers text search and navigation API which help you to create a perfect viewer.
We won’t be focusing on search this time, because it was well described in several articles (see this or this). Instead, we’ll show how to find and highlight existing links that are often used to implement quick navigation between chapters of redirect users to external web resource.
In addition, Adobe reader often parses document text and detects external links to web resources. It then allows you to click on these links, as there were link annotation objects created while actually it’s just a plain text. So you could also use text search with regular expressions to find the links on page as a second option.
In this article we’ll show how to use Apitron PDF Rasterizer and its API to convert PDF page to image and highlight all links on this page.
static void Main(string args)
// prepare graphics objects
Bitmap renderedPage = null;
Brush highlightBrush = new SolidBrush(Color.FromArgb(126, 255, 255, 0));
// store rendering settings
Resolution renderingResolution = new Resolution(144, 144);
RenderingSettings renderingSettings = new RenderingSettings();
Page firstPage = null;
// a list of rects to highlight
IList<RectangleF> highlightRects = new List<RectangleF>();
// open PDF document
using (FileStream fs = new FileStream("../../files/test.pdf",
using (Document doc = new Document(fs))
firstPage = doc.Pages;
renderedPage = firstPage.Render(renderingResolution, renderingSettings);
// parse links and store highlight rects
foreach (Link link in firstPage.Links)
Apitron.PDF.Rasterizer.Rectangle locationRect =
// search text in the same document using regular expression matching URLs
using (SearchIndex search = new SearchIndex(new FileStream("../../files/test.pdf",
// first page only
if (handlerArgs.PageIndex > 0)
handlerArgs.CancelSearch = true;
// add highlight rects by processing found items
foreach (SearchResultItem item in handlerArgs.ResultItems)
SearchResultRegion searchResultRegion =
foreach (double block in searchResultRegion.Blocks)
float xMin = float.MaxValue;
float yMin = float.MaxValue;
float xMax = float.MinValue;
float yMax = float.MinValue;
for (int i = 0; i < block.Length;)
xMin = (float)Math.Min(xMin, block[i]);
xMax = (float)Math.Max(xMax, block[i++]);
yMin = (float)Math.Min(yMin, block[i]);
yMax = (float)Math.Max(yMax, block[i++]);
// render highlighted rects
HighlightRects(renderedPage, highlightRects, highlightBrush);
Console.WriteLine("Press any key to continue...");
/// Highlights a list of rects.
private static void HighlightRects(Bitmap renderedPage,
using (Graphics g = Graphics.FromImage(renderedPage))
foreach (RectangleF rect in highlightRects)
/// Transforms PDF rect to GDI rect.
/// <param name="locationRect">Rect to transform.</param>
/// <param name="height">The height of the page.</param>
/// <returns>Transformed GDI rect.</returns>
private static RectangleF TransformToGDIRect(Rectangle locationRect, double height)
return new RectangleF((float)locationRect.Left, (float)(height - locationRect.Top),
The complete code sample can be found in our github repository.
Pic. 1 Original PDF document with links
You can see that this document has three links, two of them are marked with link annotation object and the last one is a part of the text.
Resulting image looks as follows:
Pic. 2 PDF page with highlighted links
You can see that all links are highlighted including the link that was found in text and doesn’t have a link annotation created for it.
Apitron PDF Rasterizer can be used to convert PDF to image or create custom PDF viewers, and provides comprehensive API that helps you to get the job done. It can also be used for searching text and dumping font names used in PDF document. It’s cross platform and works on many modern platforms that makes your code reusable and easily portable.