Question

Is it possible put an user control inside a doument viewer? If possible, how will it be that?

Answer 1

You can use the following..

Edit
Added a Grid which binds its Width/Height to the FixedPage ActualWidth/ActualHeight to achieve centering

<DocumentViewer>
    <FixedDocument>
        <PageContent>
            <FixedPage HorizontalAlignment="Center">
                <Grid Width="{Binding RelativeSource={RelativeSource AncestorType={x:Type FixedPage}},
                                      Path=ActualWidth}"
                      Height="{Binding RelativeSource={RelativeSource AncestorType={x:Type FixedPage}},
                                       Path=ActualHeight}">
                    <local:MyUserControl HorizontalAlignment="Center"/>
                </Grid>
            </FixedPage>
        </PageContent>
    </FixedDocument>
</DocumentViewer>

Unfortunately the Visual Studio 2010 designer is broken here and you’ll get a message saying “Property ‘pages’ does not support values of type ‘PageContent`.
This is reported here: WPF FixedDocument object doesn’t allow PageContent children

As a workaround you can load it in code behind

Xaml

<DocumentViewer>
    <FixedDocument Loaded="FixedDocument_Loaded"/>
</DocumentViewer>

Code behind

private void FixedDocument_Loaded(object sender, RoutedEventArgs e)
{
    FixedDocument fixedDocument = sender as FixedDocument;

    MyUserControl myUserControl = new MyUserControl();
    myUserControl.HorizontalAlignment = HorizontalAlignment.Center;
    myUserControl.VerticalAlignment = VerticalAlignment.Center;

    Grid grid = new Grid();            
    grid.Children.Add(myUserControl);

    FixedPage fixedPage = new FixedPage();
    fixedPage.Children.Add(grid);

    Binding widthBinding = new Binding("ActualWidth");
    widthBinding.Source = fixedPage;
    Binding heightBinding = new Binding("ActualHeight");
    heightBinding.Source = fixedPage;
    grid.SetBinding(Grid.WidthProperty, widthBinding);
    grid.SetBinding(Grid.HeightProperty, heightBinding);

    PageContent pageContent = new PageContent();
    (pageContent as IAddChild).AddChild(fixedPage);

    fixedDocument.Pages.Add(pageContent);
}