RtfFormatProvider provider = new RtfFormatProvider();
var rtf = File.ReadAllText(ofd.FileName);
rtf = this.ReplaceSelfBasedOnStyle(rtf);
this.flowDocument = provider.Import(rtf);

...

private string ReplaceSelfBasedOnStyle(string rtf)
{
    string pattern = @"{[\n]*\\s[0-9]+[^;]* \\\w* (Normal);}";

    var matches = Regex.Matches(rtf, pattern);
    foreach (Match match in matches)
    {
        string oldValue = match.Value;
        string newValue = oldValue.Replace(@"\sbasedon0 ", string.Empty);

        rtf = rtf.Replace(oldValue, newValue);
    }

    return rtf;
}