Added a new Razor page `EnvelopeReceiverPage_DxPdfViewer.razor` with a route `/envelope/DxPdfViewer`. Integrated DevExpress components, including `DxPdfViewer` for displaying PDF documents and `DxFileInput` for drag-and-drop file uploads. Styled the drag-and-drop zone with custom CSS. Initialized the viewer with a default embedded PDF file and implemented logic to handle file uploads dynamically.
