public static string fix_encoding(string src)
    {
        StringWriter return_str = new StringWriter();
        byte[] byte_array = Encoding.ASCII.GetBytes(src.Substring(0, src.Length));
        int len = byte_array.Length;
        byte byt;
        for(var i=0; i<len; i+=1)
        {
            byt = byte_array[i];
            if (byt == 63)
            {
                return_str.Write(" ");
            }
            else
            {
                return_str.Write(Encoding.ASCII.GetString(byte_array, i, 1));
            }
        }
        return return_str.ToString();
    }

StringWriter output = new StringWriter();
                output.WriteLine(PdfTextExtractor.GetTextFromPage(reader, page, new SimpleTextExtractionStrategy()));
                currentText = fix_encoding(output.ToString());

public static string remove_non_ascii(string src)
{
    return Regex.Replace(src, @"[^u0000-u007F]", " ");
}