How to detect hidden text in PDF using iTextsharper using C#

public string ReadFile(string Filename)
{
    PdfReader reader = new PdfReader(Filename);

    string pdfText = string.Empty;
    string OCRErrorPages = string.Empty;

    for (int i = 1; i <= reader.NumberOfPages; i++)
    {
        iTextSharp.text.pdf.parser.ITextExtractionStrategy its = new iTextSharp.text.pdf.parser.SimpleTextExtractionStrategy();

        String extractText = iTextSharp.text.pdf.parser.PdfTextExtractor.GetTextFromPage(reader, i, its);

        extractText = Encoding.UTF8.GetString(ASCIIEncoding.Convert(Encoding.Default, Encoding.UTF8, Encoding.Default.GetBytes(extractText)));

        if (extractText != "")
        {
            pdfText = pdfText + extractText;
        }
        else
        {
            OCRErrorPages = OCRErrorPages + i + extractText + "<br>";
        }
    }
    reader.Close();
    if (OCRErrorPages != "")
    {
        return OCRErrorPages + " This page contains no text";
    }
    else
    {
        return pdfText;
    }

}

public string ExtractText(string inFileName)
{
    string line = string.Empty;
    // Create a reader for the given PDF file
    PdfReader reader = new PdfReader(inFileName);

    int totalLen = 68;
    float charUnit = ((float)totalLen) / (float)reader.NumberOfPages;
    int totalWritten = 0;
    float curUnit = 0;

    for (int page = 1; page <= reader.NumberOfPages; page++)
    {
        line += ExtractTextFromPDFBytes(reader.GetPageContent(page)) + " ";

        var thing = reader.GetPageContent(page);

        // Write the progress.
        if (charUnit >= 1.0f)
        {
            for (int i = 0; i < (int)charUnit; i++)
            {
                Console.Write("#");
                totalWritten++;
            }
        }
        else
        {
            curUnit += charUnit;
            if (curUnit >= 1.0f)
            {
                for (int i = 0; i < (int)curUnit; i++)
                {
                    Console.Write("#");
                    totalWritten++;
                }
                curUnit = 0;
            }
        }
    }

    if (totalWritten < totalLen)
    {
        for (int i = 0; i < (totalLen - totalWritten); i++)
        {
            Console.Write("#");
        }
    }
    return line;
}

PdfContentByte.TEXT_RENDER_MODE_INVISIBLE