With Serial 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);
        scanner.nextInt();
        int count = 100;
        do {
            while (count > 0) {
                run();
                count--;
            }
            System.out.println("Iteration done!");
            count = scanner.nextInt();
        }
        while (count != 0);

        PDFNet.terminate();

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

    private static void run() throws Exception {
        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();
        }
    }
}