Include support for layers

While the other annotations are drawn on top of the content, the highlight annotation is behind the text it highlights. In other words, it has to be inserted at a specific place in the ContentElementsUILayer. The pdf seems to mark the precise place where the highlight should be placed with the BDC, MCID and EMC operators in the content. At the moment, the read-only rendering of the highlight annotation has been implemented with an instance of the AnnotationUILayer called "HighlightUILayer" which is placed behind the ContentElementsUILayer. With this implementation, depending on the contents of the pdf, the highlight annotation might be visible or it might not be. 

To reproduce:
- Drop a RadPdfViewer on a blank form at design time. 

Some documents reqire this assembly as well. 

To reproduce:
Open a document that has highlighted areas.

One should be able to handle the annotations/hyperlinks clicking. 

Add a navigation pane to view attachments under a pdf. it is possible to have various attachments under a pdf file, such has another pdf. For example, when converting an email with attachments, the resulting pdf has attachments. 

RadPdfViewer has no possibility to view those right now. Could be an event and let the calling program manage this.

For our needs, I would like an event, because our attached files are also pdfs, so I could open them in a new tab

When Find Next is used multiple times fast, and pages are still loaded asynchronously, sometimes NullReferenceException is thrown with the following call stack:

System.NullReferenceException: Object reference not set to an instance of an object.
   at Telerik.Windows.Documents.Fixed.Text.TextRecognizer.GetLastCharacter()
   at Telerik.Windows.Documents.Fixed.Text.TextRecognizer.IsLineBreak(Glyph glyph)
   at Telerik.Windows.Documents.Fixed.Text.TextRecognizer.ProcessGlyph(Glyph glyph)
   at Telerik.Windows.Documents.Fixed.Text.TextRecognizer.CreateTextPage(RadFixedPageInternal page)
   at Telerik.Windows.Documents.Fixed.Text.TextDocument.GetTextPage(Int32 index)
   at Telerik.Windows.Documents.Fixed.Search.KMPTextSearch.FindNext(TextDocument document, TextPosition startPosition, String searchText, Boolean matchCase, Boolean wholeWordsOnly)
   at Telerik.Windows.Documents.Fixed.Search.TextSearch.Find(String text, TextSearchOptions options)

...

This exception seems to be related to AES algorithm PaddingMode property. 

 Currently, the first document is loaded. 

Workaround:
Wait for the document to load before loading the new document. 

This is reproducible for images with FlateDecode and predictor value in the range between 10 and 15. As an example you may take a look at the DecodeParms property in the following image PDF dictionary:

<< /BitsPerComponent 8 /ColorSpace /DeviceRGB /DecodeParms << /BitsPerComponent 8 /Colors 3 /Columns 1024 /Predictor 15 >> /Filter /FlateDecode /Height 2868 /Subtype /Image /Type /XObject /Width 1024 /Length 1236707 >>

Overprinting is option in the GraphicState that allows combining colors when drawing one object above another object.  It is briefly described on page 284 and page 565 in PdfReference 1.7. However, the result of overprinting is device dependent and there are no strict instructions how the result should look like. This feature also includes support for the different blending modes.

There is opportunity to further optimize the caching mechanism of the image sources, which will reduce the memory consumption. The fast scrolling in a document containing big images could lead to an OutOfMemoryException.
Openining very large images as well. 

To reproduce: please run the attached sample project. If you reload the document, the horizontal scrollbar is not shown.

Workaround:
        public RadForm1()
        {
            InitializeComponent();

            this.radPdfViewer1.FitToWidth = false;
            this.radPdfViewer1.ViewerMode = Telerik.WinControls.UI.FixedDocumentViewerMode.None;
            this.radPdfViewer1.EnableThumbnails = false; 
            this.radPdfViewer1.DocumentLoaded += radPdfViewer1_DocumentLoaded;
            this.radPdfViewer1.LoadDocument(@"..\..\Go Air - Himanshe Ajeet Dubey - 17-01-2018 - Ranchi to Mumbai.pdf");
            Application.DoEvents();
        }

        private void radPdfViewer1_DocumentLoaded(object sender, EventArgs e)
        {
            Application.DoEvents();
            this.radPdfViewer1.FitToWidth = true;
        }

Load a file in RadPdfViewer, select some of the text, right click and select Copy. A sample pdf file is attached.