Workaround: until the feature be implemented one can print each page individually respecting its orientation whether it is portrait or landscape
public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
 
        this.radPdfViewer1.LoadDocument(@"..\..\sample.pdf");
    }
 
    private void radButton1_Click(object sender, EventArgs e)
    {
        RadPrintDocument doc = new RadPrintDocument();
        doc.AssociatedObject = this.radPdfViewer1;
 
        for (int i = 0; i < this.radPdfViewer1.Document.Pages.Count; i++)
        {
            RadFixedPage page = this.radPdfViewer1.Document.Pages[i];
 
            doc.CurrentPage = i;
            doc.PrinterSettings.PrintRange = PrintRange.CurrentPage;
            doc.Landscape = page.ActualWidth > page.ActualHeight;
 
            doc.Print();
        }
    }
}

To reproduce:
- Print a document that fills the entire page.
- On the print preview dialog the page is centered however when printed it is not.

In addition, one should be able to control the page scaling. 

at System.Drawing.Drawing2D.GraphicsPath.Clone()
   at Telerik.WinControls.PdfViewer.PdfElementsRenderer.RenderGlyph(Glyph glyph)
   at Telerik.WinControls.PdfViewer.FixedPageRenderer.DrawGlyph(Glyph glyph, Graphics graphics)
   at Telerik.WinControls.PdfViewer.FixedPageRenderer.DrawContentElement(IContentElement contentElement, Graphics graphics)
   at Telerik.WinControls.PdfViewer.FixedPageRenderer.DrawGlyphsLayoutBox(GlyphsLayoutBox glyphBox, Graphics graphics)
   at Telerik.WinControls.PdfViewer.FixedPageRenderer.DrawContentElement(IContentElement contentElement, Graphics graphics)
   at Telerik.WinControls.PdfViewer.FixedPageRenderer.DrawContainer(Container container, Graphics graphics)
   at Telerik.WinControls.PdfViewer.FixedPageRenderer.DrawContentElement(IContentElement contentElement, Graphics graphics)
   at Telerik.WinControls.PdfViewer.FixedPageRenderer.DrawContainer(Container container, Graphics graphics)
   at Telerik.WinControls.PdfViewer.FixedPageRenderer.DrawContentElement(IContentElement contentElement, Graphics graphics)
   at Telerik.WinControls.PdfViewer.FixedPageRenderer.DrawPage(ContentCollection content, Graphics graphics, Matrix pageTransform)
   at Telerik.WinControls.UI.RadFixedPageElement.pageRenderer_DoWork(Object sender, DoWorkEventArgs e)
   at System.ComponentModel.BackgroundWorker.OnDoWork(DoWorkEventArgs e)
   at System.ComponentModel.BackgroundWorker.WorkerThreadStart(Object argument)
   at System.Drawing.Drawing2D.GraphicsPath.Clone()
   at Telerik.WinControls.PdfViewer.PdfElementsRenderer.RenderGlyph(Glyph glyph)
   at Telerik.WinControls.PdfViewer.FixedPageRenderer.DrawGlyph(Glyph glyph, Graphics graphics)
   at Telerik.WinControls.PdfViewer.FixedPageRenderer.DrawContentElement(IContentElement contentElement, Graphics graphics)
   at Telerik.WinControls.PdfViewer.FixedPageRenderer.DrawGlyphsLayoutBox(GlyphsLayoutBox glyphBox, Graphics graphics)
   at Telerik.WinControls.PdfViewer.FixedPageRenderer.DrawContentElement(IContentElement contentElement, Graphics graphics)
   at Telerik.WinControls.PdfViewer.FixedPageRenderer.DrawContainer(Container container, Graphics graphics)
   at Telerik.WinControls.PdfViewer.FixedPageRenderer.DrawContentElement(IContentElement contentElement, Graphics graphics)
   at Telerik.WinControls.PdfViewer.FixedPageRenderer.DrawContainer(Container container, Graphics graphics)
   at Telerik.WinControls.PdfViewer.FixedPageRenderer.DrawContentElement(IContentElement contentElement, Graphics graphics)
   at Telerik.WinControls.PdfViewer.FixedPageRenderer.DrawPage(ContentCollection content, Graphics graphics, Matrix pageTransform)
   at Telerik.WinControls.UI.RadFixedPageElement.pageRenderer_DoWork(Object sender, DoWorkEventArgs e)
   at System.ComponentModel.BackgroundWorker.OnDoWork(DoWorkEventArgs e)
   at System.ComponentModel.BackgroundWorker.WorkerThreadStart(Object argument)

1- the load of a document take a while to load (if successed).
2- An unhandled exception thrown while loading a document (Out of memory or Invalid parameter)

the file that i'm trying to load is attached and created using Telerik Document Processing Library.

************** Exception Text **************
System.ArgumentException: Parameter is not valid.
   at System.Drawing.Bitmap..ctor(Int32 width, Int32 height, PixelFormat format)
   at System.Drawing.Bitmap..ctor(Image original, Int32 width, Int32 height)
   at System.Drawing.Bitmap..ctor(Image original)
   at Telerik.WinControls.PdfViewer.PdfElementsRenderer.RenderImage(ImageSource source, ResourceKey key, BackgroundWorker cancellationToken)
   at Telerik.WinControls.PdfViewer.FixedPageRenderer.DrawImage(Image image, Graphics graphics)
   at Telerik.WinControls.PdfViewer.FixedPageRenderer.DrawContentElement(IContentElement contentElement, Graphics graphics)
   at Telerik.WinControls.PdfViewer.FixedPageRenderer.DrawContainer(Container container, Graphics graphics)
   at Telerik.WinControls.PdfViewer.FixedPageRenderer.DrawContentElement(IContentElement contentElement, Graphics graphics)
   at Telerik.WinControls.PdfViewer.FixedPageRenderer.DrawContainer(Container container, Graphics graphics)
   at Telerik.WinControls.PdfViewer.FixedPageRenderer.DrawContentElement(IContentElement contentElement, Graphics graphics)
   at Telerik.WinControls.PdfViewer.FixedPageRenderer.DrawPage(ContentCollection content, Graphics graphics, Matrix pageTransform)
   at Telerik.WinControls.UI.FixedPagePreRenderer.rendererWorker_DoWork(Object sender, DoWorkEventArgs e)
   at System.ComponentModel.BackgroundWorker.OnDoWork(DoWorkEventArgs e)
   at System.ComponentModel.BackgroundWorker.WorkerThreadStart(Object argument)

Thank you

PDF Export needs to support certificate signatures so the end user of the document knows the document hasn't been changed.

Workaround: print the document page by page:
private void radButton1_Click(object sender, EventArgs e)
{
    RadPrintDocument d = new RadPrintDocument();
    d.PrinterSettings.PrintRange = PrintRange.CurrentPage;
    d.AssociatedObject = this.radPdfViewer1;
    for (int i = 0; i < this.radPdfViewer1.Document.Pages.Count; i++)
    {
        d.CurrentPage = i;
        d.Print();
    }
}

Note: If you use the next button to navigate to the next search result, the user should be notified when the end of the document is reached as well.

Workaround:

public Form1()
{
    InitializeComponent(); 
    this.radPdfViewer1.LoadDocument(@"..\..\..\samplePdf.pdf"); 

    //create a new button
    CommandBarButton findNext = new CommandBarButton();
    findNext.Text = PdfViewerLocalizationProvider.CurrentProvider.GetLocalizedString(PdfViewerStringId.NavigatorFindNextButton);
    findNext.Click += findNext_Click;
    findNext.Image = this.radPdfViewerNavigator1.FindNextButton.Image;
    //hide the currently available button
    this.radPdfViewerNavigator1.DefaultStrip.Items.Add(findNext);
    this.radPdfViewerNavigator1.FindNextButton.Visibility = ElementVisibility.Collapsed;
}

private void findNext_Click(object sender, EventArgs e)
{
    if (this.radPdfViewerNavigator1.SearchTextBox.Text != string.Empty)
    { 
        if (this.radPdfViewerNavigator1.AssociatedViewer == null || this.radPdfViewerNavigator1.AssociatedViewer.Document == null)
        {
            return;
        }

        PropertyInfo internalDocumentPI = typeof(RadFixedDocument).GetProperty("InternalDocument", BindingFlags.NonPublic | BindingFlags.Instance);
        object internalDocument = internalDocumentPI.GetValue(this.radPdfViewerNavigator1.AssociatedViewer.Document, null);
        object textSearch = internalDocument.GetType().GetProperty("TextSearch", BindingFlags.NonPublic | BindingFlags.Instance).GetValue(internalDocument, null);
        MethodInfo mi = textSearch.GetType().GetMethod("Find", BindingFlags.Public | BindingFlags.Instance);
        TextSearchOptions searchOptions = new TextSearchOptions(false);
        SearchResult searchResult = mi.Invoke(textSearch, new object[2] { this.radPdfViewerNavigator1.SearchTextBox.Text, searchOptions }) as SearchResult;
        if (searchResult.Result != null)
        {
            this.radPdfViewerNavigator1.AssociatedViewer.Select(searchResult);
        }
        else
        {
            RadMessageBox.Show("No matches");
        }
    }
}