PDFTextStripper not working with multiple threads

 private void scanFolderFiles(File scanDirectory) {
        File[] filesScan = scanDirectory.listFiles();

        if ( filesScan.length > 0 ) {
            int iterator=0;
            for (final File fileEntry : filesScan) {

           
                if (fileEntry.isDirectory()) {
                    scanFolderFiles(fileEntry);
                } else {
                    try {
                       new PDFExtractThread(fileEntry).start();
                    }
                    catch (Exception e) {
                        e.printStackTrace();
                    }
                }

                iterator++;
            }
        }
    }


////////////////////////////////////////////////////////////////////////////////////////////
package org.pdfextractor;


import org.apache.pdfbox.pdmodel.PDDocument;
import org.apache.pdfbox.text.PDFTextStripper;
import java.io.*;

/**
 * Created by dru on 21.11.2016.
 */
class PDFExtractThread extends Thread {

    private String fileName;
    private File readFile;
    private PDDocument document;

    public PDFExtractThread(File readFile) {

        try {

            this.readFile = readFile;

        } catch (Exception e) {
            e.printStackTrace();
            System.exit(1);
        }
    }


    public void run() {

        try {

            //get FileName
            this.readFile.setWritable(true);
            this.document = PDDocument.load(this.readFile);
            this.fileName = (new String(this.readFile.getName()).toLowerCase().replace(".pdf", ""));

            pdfBoxExtractText();
            //Closing the document
            this.document.close();

        } catch (IOException e) {
            e.printStackTrace();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }


    public void pdfBoxExtractText() throws Exception {
        //Retrieving text from PDF document
        PDFTextStripper pdfStripper = new PDFTextStripper();
        System.out.println(this.fileName);
        FileWriter fileWriter = new FileWriter(this.fileName+".txt");
        BufferedWriter writer = new BufferedWriter(fileWriter);
        String text = pdfStripper.getText(this.document);

        writer.write(text);
        writer.close();
    }
}