With Parallel Processing

import com.pdftron.pdf.PDFDoc;
import com.pdftron.pdf.PDFNet;

import javax.annotation.PostConstruct;
import javax.annotation.PreDestroy;
import java.io.FileNotFoundException;
import java.util.ArrayList;
import java.util.Enumeration;
import java.util.List;
import java.util.zip.ZipEntry;
import java.util.zip.ZipFile;
import java.util.stream.Collectors;
import java.util.Scanner;

public class PDFTron {
    public static void main(String[] args) throws Exception {
        PDFNet.initialize();
        Scanner scanner = new Scanner(System.in);

        int count = 20;
        while (count > 0) {
            new Thread(new B()).start();
            count--;
        }

        System.out.println("Done");
        scanner.nextInt();
        PDFNet.terminate();
        scanner.nextInt();
    }

    private static class B implements Runnable {
        public void run() {
            int count = 10;
            while (count > 0) {
                try {
                    String pathToZipFile = "files.zip";
                    ZipFile zipFile = new ZipFile(pathToZipFile);

                    List<PDFDoc> pdfDocs = zipFile.stream().parallel()
                            .map(zipEntry -> {
                                try {
                                    return new PDFDoc(zipFile.getInputStream(zipEntry));
                                } catch (Exception e) {
                                    e.printStackTrace();
                                }
                                return null;
                            })
                            .collect(Collectors.toList());

                    zipFile.close();
                    for (PDFDoc pdfDoc : pdfDocs) {
                        pdfDoc.close();
                    }

                } catch (Exception e) {
                    e.printStackTrace();
                }
                count--;
            }
        }
    }
}