Ok i found out what i was doing wrong:"Apparently the inner-exception was: Cannot convert string to URI.

So here is the corrected sample:

Code:
<Window x:Class="FlowDocTest.MainWindow"
        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:sys="clr-namespace:System;assembly=System"
        xmlns:local="clr-namespace:FlowDocTest"
        mc:Ignorable="d"
        Title="MainWindow" Height="350" Width="525">
     <Grid>
        <Grid.Resources>
            <sys:Uri x:Key="strHTML">D:\Manual.html</sys:Uri>
        </Grid.Resources>
        <WebBrowser Source="{StaticResource strHTML}"/>
    </Grid>
</Window>