How to fire exception prevent c# app from exit

Question

My app exit while executing an function from ironocr library (ocr.read()) without throw any exceptions although i put the line of code in try catch block, My question is how i throw exception to see the reason of the function failed or skip reading that file and don't exit the app. :

private void SearchPDF(string fileName)
{
    try
    {
        StringBuilder text = new StringBuilder();
        if (new FileInfo(fileName).Length == 0)
        {
            return;
        }

        var Ocr = new IronTesseract();

        Ocr.Language = OcrLanguage.English;
        using (var Input = new OcrInput(fileName))
        {
            var Result = Ocr.Read(Input);
            string currentSubText = Result.Text;
            text.Append(currentSubText);

        }
        string searchWord = "";
        txtSearch.Invoke((MethodInvoker)delegate { searchWord = txtSearch.Text.ToLower(); });
        string fileData = text.ToString();
        string fileNameWithExt = System.IO.Path.GetFileName(fileName);
        if (isIN(fileNameWithExt, fileData, searchWord))
        {
            matchingPdfFiles.Add(fileNameWithExt, fileName);
            fillInTable(fileName);
        }
    }
    catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
}