I have a Java method that generates PDFs from HTML templates using the iText 7 library. The method works fine for generating a small number of PDFs, but when I try to run this code with 10 threads and generating a large number (e.g., 1000), I encounter an out-of-memory error.Heap size increasing upto 1GB.
And also i am using a base64 String of Signature and Letterhead images.

Java

public byte[] startPdfCreation(String template, PdfConfiguration configuration, Map<String, String> notice) {
    try {
      ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
      PdfWriter writer = new PdfWriter(outputStream);
      PdfDocument pdfDocument = new PdfDocument(writer);
      pdfDocument.setDefaultPageSize(PageSize.A4);
      Document document = new Document(pdfDocument);
      document.setLeftMargin(26);
      document.setRightMargin(26);

      String letterhead = getLetterheadImage(configuration);
      pdfDocument.addEventHandler(PdfDocumentEvent.END_PAGE, new BackgroundImageHandler(letterhead));
      String signature = getSignatureImage(configuration);
      ConverterProperties properties = getConverterProperties();
      String substitutedTemplate = substituteValues(template, notice);

      List<IElement> elements = HtmlConverter.convertToElements(substitutedTemplate, properties);
      for (IElement element : elements) {
        document.add((BlockElement) element);
      }
      document.close();
      pdfDocument.close();
      return outputStream.toByteArray();
    } catch (Exception e) {
      log.error("error while creating a pdf", e);
    }
    return new byte[0];
  }

Here i am using executor service to run in 10 threads

ExecutorService executor = Executors.newFixedThreadPool(threadCount);
    CompletableFuture<Void> allTasks = CompletableFuture.allOf(
        noticeDataList.stream()
            .map(notice -> CompletableFuture.runAsync(() -> {
              PdfConfiguration pdfConfiguration = createPdfConfiguration(notice);
              byte[] fileContent = startPdfCreation(template, pdfConfiguration, notice);
              Map<String, String> uploadResponse = uploadPdf(fileContent, notice);
              if (uploadResponse != null) {
                successFiles.add(notice);
              } else {
                failedFiles.add(notice);
              }
            }, executor))
            .toArray(CompletableFuture[]::new)
    );
    return allTasks.thenApply(result -> {
      executor.shutdown();
      return new PdfGenerateResponse(successFiles, failedFiles);
    }).join();

Is there is something i am missing or what please help me out
Thanks.

How i can create upto 1000 pdf using Threads in minimum heap memory usage.