RadPdfViewer provides an API for exporting a PDF document page to an image. This can be done with the ThumbnailFactory class. The class exposes a single method - CreateThumbnail - which accepts objects of type RadFixedPage and Size, and returns an ImageSource object.
Example 1: Using the ThumbnailFactory's method
ThumbnailFactory thumbnailFactory = new ThumbnailFactory();
ImageSource imageSource = thumbnailFactory.CreateThumbnail(radFixedPageInstance, new Size(150, 250));
The ThumbnailsConverter class is a standard converter that implements the native IValueConverter. It accepts a RadFixedDocument as a value and returns a ThumbnailsCollection object. The items in the collection are of type Thumbnail and they expose information about the converted RadFixedPage and the image result. You can find a runnable example demonstrating the ThumbnailsConverter in the PdfViewer/Thumbnails SDK example in GitHub.
The converter is defined in the Telerik.Windows.Documents.Converters namespace.