Example

public class FlowDocCustomPaginator : DocumentPaginator
{
    private Size _pageSize;
    private DocumentPaginator _paginator;

    public FlowDocCustomPaginator(DocumentPaginator paginator, Size pageSize)
    {
        this._pageSize = pageSize;
        this._paginator = paginator;
    }

    public override DocumentPage GetPage(int pageNumber)
    {
        DocumentPage page = _paginator.GetPage(pageNumber);
        Grid grid = new Grid();

        //add header element
        TextBlock tb = new TextBlock();
        tb.Text = "Health Article";
        tb.FontSize = 22;
        tb.Foreground = new SolidColorBrush(Colors.LightGray);
        tb.Margin = new Thickness(30, 30, 30, 0);
        grid.Children.Add(tb);

        //add page
        grid.Children.Add(page.Visual);

        //add footer element
        Grid footer = new Grid();
        footer.VerticalAlignment = VerticalAlignment.Bottom;
        footer.Margin = new Thickness(50, 0, 50, 30);

        TextBlock copyright = new TextBlock();
        copyright.Foreground = new SolidColorBrush(Colors.Gray);
        copyright.Text = "Copyright © My Publisher Corp 2011.";
        footer.Children.Add(copyright);

        TextBlock pgNumber = new TextBlock();
        int cnt = pageNumber + 1;
        pgNumber.Text = cnt.ToString();
        pgNumber.Foreground = new SolidColorBrush(Colors.Gray);
        pgNumber.HorizontalAlignment = HorizontalAlignment.Right;
        footer.Children.Add(pgNumber);
        grid.Children.Add(footer);

        grid.Measure(_paginator.PageSize);
        grid.Arrange(new Rect(new Point(0, 0), _paginator.PageSize));

        return new DocumentPage(grid, _paginator.PageSize, page.BleedBox, page.ContentBox);
    }

    public override Size PageSize
    {
        get { return _pageSize; }
        set { _pageSize = value; }
    }

    public override bool IsPageCountValid
    {
        get
        {
            return _paginator.IsPageCountValid;
        }
    }

    public override int PageCount
    {
        get
        {
            return _paginator.PageCount;
        }
    }

    public override IDocumentPaginatorSource Source
    {
        get
        {
            return _paginator.Source;
        }
    }
}

<UserControl x:Class="TestProject.TestCustomPaginator"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:Intersoft="http://intersoft.clientui.com/schemas"
    mc:Ignorable="d"
    d:DesignHeight="800" d:DesignWidth="1000">

    <Grid x:Name="LayoutRoot" Background="White">
        <Intersoft:UXFlowDocumentViewer x:Name="Viewer1">
            <Intersoft:FlowDocument>
                <Intersoft:Paragraph>
                    <Intersoft:Bold FontSize="40">
                        <Intersoft:Run >
                            Danger Of Salt
                        </Intersoft:Run>
                    </Intersoft:Bold>
                    <Intersoft:LineBreak/>
                    <Intersoft:Run FontSize="20" Foreground="#FF5D5D5D">
                        THE DEADLY SUBSTANCE YOU CAN'T LIVE WITHOUT
                    </Intersoft:Run>
                    <Intersoft:LineBreak/>
                    <Intersoft:Run FontSize="18" Foreground="Gray">
                        BY DAVE GOLOKHOV
                    </Intersoft:Run>
                </Intersoft:Paragraph>
                <Intersoft:Section LineSpacingType="OneAndHalfLines" TextAlignment="Justify" SpacingBefore="30">
                    <Intersoft:Paragraph>
                        <Intersoft:Run>
                            Spike TV's 1000 Ways To Die shows us there's many ways to go, but according to the British Medical Journal, the real serial killer sits in a shaker on almost every table in North America. Their latest study aims to encourage the United Nations to make stiffer worldwide regulations on salt usage, which happens to be second-most cost-effective way of cutting global medical costs next to controlling tobacco use.
                        </Intersoft:Run>
                    </Intersoft:Paragraph>
                    <Intersoft:Paragraph SpacingBefore="20">
                        <Intersoft:Run>
                            In the United States alone, cutting salt intake by a third would save tens of thousands of lives per year and shave $24 billion off of health care costs. Reducing the country's intake of salt could also reduce the deficit.
                        </Intersoft:Run>
                    </Intersoft:Paragraph>
                    <Intersoft:Paragraph SpacingBefore="20">
                        <Intersoft:Run>
                            The equation is fairly simple: salt intake is directly correlated to blood pressure. In turn, high blood pressure is linked to strokes and heart disease, which is the leading cause of death in developed countries.
                        </Intersoft:Run>
                    </Intersoft:Paragraph>
                    <Intersoft:Paragraph SpacingBefore="20">
                        <Intersoft:Run>
                            Sounds serious enough, but try convincing someone to trade in their salty French fries and pretzels for an abstract concept of better health down the road. The correlation between over-salting and death isn’t as direct as a gunshot or a car accident, but the science is solid: The white stuff will kill you if you’re not careful.
                        </Intersoft:Run>
                    </Intersoft:Paragraph>
                </Intersoft:Section>
            </Intersoft:FlowDocument>
        </Intersoft:UXFlowDocumentViewer>
    </Grid>
</UserControl>

public partial class TestCustomPaginator : UserControl
{
    public TestCustomPaginator()
    {
        InitializeComponent();

        DocumentPaginator paginator = new FlowDocCustomPaginator(this.Viewer1.Document.DocumentPaginator, this.Viewer1.Document.DocumentPaginator.PageSize);
        this.Viewer1.Document.CustomPaginator = paginator;
    }
}