import com.spire.pdf.PdfDocument;

public class expandBookmarks {
    public static void main(String[] args) {

        PdfDocument doc = new PdfDocument();
        doc.loadFromFile("Sample.pdf");

        //Set true to expand all bookmarks; set false to collapse all bookmarks
        doc.getViewerPreferences().setBookMarkExpandOrCollapse(true);

        doc.saveToFile("output/expandAllBookmarks_out.pdf");
        doc.close();
    }
}

import com.spire.pdf.PdfDocument;
import com.spire.pdf.bookmarks.*;

public class expandSpecificBookmarks {
    public static void main(String[] args) {

        PdfDocument doc = new PdfDocument();
        doc.loadFromFile("Sample.pdf");

        //Set BookMarkExpandOrCollapse as "true" for the first bookmarks
        doc.getBookmarks().get(0).setExpandBookmark(true);

        //Set BookMarkExpandOrCollapse as "false" for the first level of the second bookmarks
        PdfBookmarkCollection pdfBookmark = doc.getBookmarks().get(1);
        pdfBookmark.get(0).setExpandBookmark(false);

        doc.saveToFile("output/expandSpecificBookmarks_out.pdf");
        doc.close();
    }
}