Currently, the Line breaks <br> are not exported to plain text format.
Workaround:Replace <br> tags in the HTML document with a marker
string html = File.ReadAllText("Source.html");
string newHtml = html.Replace("<br>", "[br]");
File.WriteAllText("NewSource.html", newHtml);
Then import the edited HTML and export it as plain text, then replace the markers with "\r\n"
using (Stream stream = File.OpenRead("NewSource.html"))
{
HtmlFormatProvider htmlFormatProvider = new HtmlFormatProvider();
flowDocument = htmlFormatProvider.Import(stream);
TxtFormatProvider txtFormatProvider = new TxtFormatProvider();
string text = txtFormatProvider.Export(flowDocument);
string newText = text.Replace("[br]", "\r\n");
}