The class ReadFile needs to receive a PrintWriter parameter, resp.getWriter().
The character encoding was not handled both on reading the file, as on presenting it in the browser. Here I elected for both UTF-8.
For the file it might be Charset.forName("Windows-1252") or such.
As the java Files class has everything that ReadFile could do, I used that.
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException 
{
    resp.setContentType("text/plain");
    resp.setCharacterEncoding("UTF-8");
    PrintWriter out = resp.getWriter();
    Path path = Paths.get("D:\\text.txt");
    Files.lines(path, StandardCharsets.UTF_8)
        .forEach(out::println));
}
Or using HTML formatting:
    resp.setContentType("text/html");
    resp.setCharacterEncoding("UTF-8");
    PrintWriter out = resp.getWriter();
    out.println("<!DOCTYPE html>");
    out.println("<html><head>");
    out.println("<meta charset='UTF-8'>");
    out.println("<title>The text<title>");
    out.println("</head><body><pre>");
    Path path = Paths.get("D:\\text.txt");
    Files.lines(path, StandardCharsets.UTF_8)
        .forEach(line -> {
             line = line.replace("&", "&")
                 .replace("<", "<")
                 .replace(">", ">");
             out.printl(line);
        });
    out.println("</pre></body></html>");
With using a ReadFile class:
public class  ReadFile {
    public void  readFile(PrintWriter out) throws IOException {
        Path path = Paths.get("D:\\text.txt");
        Files.lines(path, StandardCharsets.UTF_8)
            .forEach(out::println));
    }
    public static void main(String[] args) throws IOException {   
        ReadFile read = new ReadFile();
        read.readFile(new PrintWriter(System.out));
    }
}
protected void doGet(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException 
{
    resp.setContentType("text/plain");
    resp.setCharacterEncoding("UTF-8");
    PrintWriter out = resp.getWriter();
    ReadFile read = new ReadFile();
    read.readFile(System.out);
}