FlowDocument flowDoc = new FlowDocument();
using (MemoryStream mem = new MemoryStream(bytesFromDB)
            {
                TextRange textRange = new TextRange(
                    flowDoc.ContentStart,
                    flowDoc.ContentEnd);
                textRange.Load(mem, DataFormats.XamlPackage);
            }

textRange.Load(mem, DataFormats.XamlPackage);

var sw = Stopwatch.StartNew();
var doc = new FlowDocument();
var tr = new TextRange(doc.ContentStart, doc.ContentEnd);
using (var stream = File.OpenRead(@"D:CSharp Language Specification.rtf"))
    tr.Load(stream, DataFormats.Rtf);
Viewer.Document = doc;
sw.Stop();
Status.Text = "Finished in " + sw.Elapsed;

try
{
    var sw = Stopwatch.StartNew();
    Viewer.Document = null;
    Status.Text = "Loading...";
    Progress<string> p = new Progress<string>(s => Status.Text = s);
    var xaml = await Task.Run(
        () => LoadAsXamlInBackground(@"D:CSharp Language Specification.rtf", p));
    Status.Text = "Rendering...";
    var doc = (FlowDocument)XamlReader.Load(xaml);
    Viewer.Document = doc;
    sw.Stop();
    Status.Text = "Finished in " + sw.Elapsed;
}
catch (Exception ex)
{
    Status.Text = "Failed: " + ex.ToString();
}

// ...
MemoryStream LoadAsXamlInBackground(string filename, IProgress<string> progress)
{
    var ms = new MemoryStream();
    var doc = new FlowDocument();
    var tr = new TextRange(doc.ContentStart, doc.ContentEnd);
    using (var stream = File.OpenRead(filename))
        tr.Load(stream, DataFormats.Rtf);
    progress.Report("Loaded, converting...");
    XamlWriter.Save(doc, ms);
    progress.Report("Converted...");
    ms.Position = 0;
    return ms;
}

Main

Thread th = new Thread ( delegate () { load (); } );
  th.TrySetApartmentState ( ApartmentState.STA );
  th.Start ();