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.

It will be a great addition of RadPdfViewer if it supports forms/controls.

The events should provide information as to when a page is about to be rendered and when it is displayed in the control.

How to reproduce: check the attached project

Workaround: manually unload the document before disposing the control, the attached project includes a solution

This functionality should be similar to the one provided by Adobe.