java.sql.SQLException: Operation not allowed after ResultSet closed
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:964)
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:897)
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:886)
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:860)
    at com.mysql.jdbc.ResultSetImpl.checkClosed(ResultSetImpl.java:743)
    at com.mysql.jdbc.ResultSetImpl.next(ResultSetImpl.java:6301)
    at reportpdf.ReportPDF.main(ReportPDF.java:84)

import com.itextpdf.text.Chunk;
    import com.itextpdf.text.Document;
    import com.itextpdf.text.DocumentException;
    import com.itextpdf.text.Font;
    import com.itextpdf.text.Paragraph;
    import com.itextpdf.text.Phrase;
    import com.itextpdf.text.pdf.PdfPCell;
    import com.itextpdf.text.pdf.PdfPTable;
    import com.itextpdf.text.pdf.PdfWriter;
    import com.mysql.jdbc.Connection;

    import java.io.FileNotFoundException;
    import java.io.FileOutputStream;
    import java.sql.DriverManager;
    import java.sql.ResultSet;
    import java.sql.SQLException;
    import java.sql.Statement;
    import java.time.LocalDate;
    import java.util.Date;

    public class ReportPDF {
    private static Font headerFontBold = new                                 Font(Font.FontFamily.UNDEFINED,14,Font.BOLD);
    private static Font headerFont = new Font(Font.FontFamily.UNDEFINED,12);
    private static Font titleFont = new Font(Font.FontFamily.UNDEFINED,12,Font.BOLD);
    private static LocalDate localDate = LocalDate.now();
    private static String output = ""+localDate;

    public static void main(String[] args) {
    Document document = new Document();
    try {
    PdfWriter.getInstance(document,new FileOutputStream("Inventory Report "+output+".pdf"));
        document.open();

        Paragraph para1 = new Paragraph("UNIVERSITY OF SANTO TOMAS", headerFontBold);
        para1.setAlignment(Paragraph.ALIGN_CENTER);
        para1.setSpacingAfter(2);
        document.add(para1);

        Paragraph para2 = new Paragraph("España, Manila", headerFont);
        para2.setAlignment(Paragraph.ALIGN_CENTER);
        para2.setSpacingAfter(2);
        document.add(para2);

        Paragraph para3 = new Paragraph("IICS Robotics Laboratory Inventory Report", headerFont);
        para3.setAlignment(Paragraph.ALIGN_CENTER);
        para3.setSpacingAfter(20);
        document.add(para3);

        Paragraph para4 = new Paragraph("Inventory Report as of: "+  new Date(), titleFont);
        para4.setAlignment(Paragraph.ALIGN_LEFT);
        para4.setSpacingAfter(2);
        document.add(para4);

        Paragraph para5 = new Paragraph("Generated by: ", titleFont);
        para5.setAlignment(Paragraph.ALIGN_LEFT);
        para5.setSpacingAfter(2);
        document.add(para5);

        Paragraph para6 = new Paragraph("Generated for date/s: ", titleFont);
        para6.setAlignment(Paragraph.ALIGN_LEFT);
        para6.setSpacingAfter(20);
        document.add(para6);

        try{
        Class.forName("com.mysql.jdbc.Driver").newInstance();
        Connection  con=(Connection) DriverManager.getConnection ("jdbc:mysql://localhost:3306/pdfreport?autoReconnect=true&useSSL=false", "root", "root");
        Statement st = (Statement) con.createStatement();
        ResultSet rs = st.executeQuery("select id, firstName, lastName, emailAddress, birthday, age from pdfreport.registration;");

        PdfPTable recordTable = new PdfPTable(6);
        PdfPCell recordCell;

        while (rs.next()) {
                String id = rs.getString("id");
                recordCell=new PdfPCell(new Phrase(id));
                recordTable.addCell(recordCell);

                String firstName=rs.getString("firstName");
                recordCell=new PdfPCell(new Phrase(firstName));
                recordTable.addCell(recordCell);

                String lastName=rs.getString("lastName");
                recordCell=new PdfPCell(new Phrase(lastName));
                recordTable.addCell(recordCell);

                String emailAddress=rs.getString("emailAddress");
                recordCell=new PdfPCell(new Phrase(emailAddress));
                recordTable.addCell(recordCell);

                String birthday=rs.getString("birthday");
                recordCell=new PdfPCell(new Phrase(birthday));
                recordTable.addCell(recordCell);

                String age=rs.getString("age");
                recordCell=new PdfPCell(new Phrase(age));
                recordTable.addCell(recordCell);

                document.add(recordTable);
                rs.close();
                st.close();
                con.close();
                document.close();

            }

        }
        catch (ClassNotFoundException | IllegalAccessException | InstantiationException | SQLException e){
            e.printStackTrace();
        }
        document.close();
    } catch (DocumentException | FileNotFoundException e) {
        e.printStackTrace();
    }
}