using (Stream stream = ofd.OpenFile())
{
	this.pdfViewer.DocumentSource = new PdfDocumentSource(stream);
}

using (Stream stream = ofd.OpenFile())
{
	PdfImportSettings importSettings = new PdfImportSettings
	{
		CopyStream = true
	};

	this.pdfViewer.DocumentSource = new PdfDocumentSource(stream, importSettings);
}

<telerik:CurrentPageTextBox x:Name="PART_CurrentPageNumberTextBox" 
							KeyDown="PART_CurrentPageNumberTextBox_KeyDown"
							Text="{Binding FixedDocumentViewer.CurrentPageNumber, Mode=TwoWay}"/>

private void PART_CurrentPageNumberTextBox_KeyDown(object sender, System.Windows.Input.KeyEventArgs e)
{
	TextBox textBox = sender as TextBox;
	if (textBox != null)
	{
		if (e.Key == System.Windows.Input.Key.Enter)
		{
			int pagesCount = this.pdfViewer.PagesCount;
			int currentPageNumber = this.pdfViewer.CurrentPageNumber;
			if (this.pdfViewer.Document != null && (currentPageNumber < 1 || currentPageNumber > pagesCount))
			{
				RadFixedPage currentPage = this.pdfViewer.CurrentPage;
				int pageNum = this.pdfViewer.Document.Pages.IndexOf(currentPage) + 1;
				this.pdfViewer.CurrentPageNumber = pageNum;
			}
		}
	}
}