如何将List<PdfDocument>合并为单个PdfDocument

import com.itextpdf.kernel.pdf.PdfDocument;
import com.itextpdf.kernel.pdf.PdfMerger;

import java.util.List;

public class PdfMergeUtil {
    public static PdfDocument mergePdfDocuments(List<PdfDocument> pdfDocuments) {
        PdfDocument mergedPdfDocument = new PdfDocument(new PdfWriter("merged.pdf"));
        PdfMerger merger = new PdfMerger(mergedPdfDocument);

        for (PdfDocument pdfDocument : pdfDocuments) {
            int numberOfPages = pdfDocument.getNumberOfPages();

            for (int i = 1; i <= numberOfPages; i++) {
                merger.merge(pdfDocument, i, i);
            }

            pdfDocument.close();
        }

        mergedPdfDocument.close();

        return mergedPdfDocument;
    }
}