The performance of loading the thumbnails is decreased with the removing of the cache in PdfViewer. 

Possible reason is that the pages are no longer loaded asynchronously.

Steps to reproduce:
- Open the SDK with the thumbnails for PDFViewer.
- Notice that the load of the thumbnails is slow.

The attached image shows the issue.

As a result, the glyphs are not measured and positioned properly. The issue applies to the TrueType and Type1 fonts.

Layers represent groups of optional content which can be hidden or shown.

Changing the value of the DocumentSource property at runtime causes the RadPdfViewer to display a busy indicator and not update the displayed PDF file with the new one.

To work this around, derive from the RadPdfViewer and create a new dependency property for the property of the type of PdfDocumentSource (for example, from your view model). In its callback method, call the SetDocumentSource method of the derived RadPdfViewer by passing the new value:

public class MyPdfViewer : RadPdfViewer
{
    public PdfDocumentSource PdfDocumentSource
    {
        get { return (PdfDocumentSource)GetValue(PdfDocumentSourceProperty); }
        set { SetValue(PdfDocumentSourceProperty, value); }
    }

    public static readonly DependencyProperty PdfDocumentSourceProperty =
        DependencyProperty.Register("PdfDocumentSource", typeof(PdfDocumentSource), typeof(MyPdfViewer), new PropertyMetadata(new PropertyChangedCallback(OnDocumentSourceChanged)));

    private static void OnDocumentSourceChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        MyPdfViewer viewer = (MyPdfViewer)d;

        viewer.SetDocumentSource((PdfDocumentSource)e.NewValue);
    }
}

<local:MyPdfViewer x:Name="pdfViewer" PdfDocumentSource="{Binding MyViewModelPropertyOfTypePdfDocumentSource}" />

The default WPF PrintDialog cannot print asynchronously when it is shown in UI. However, you may see the demo in this forum post showing how to create custom PrintDialog and perform async printing with it:
http://www.telerik.com/forums/pdfviewer-print-makes-ui-unresponsive#js0YdzFWc0Oa8C3g6a18lg