@Controller
@RequestMapping("/mail")
public class MailController {

    @Autowired
    public EmailService emailService;


    @RequestMapping(value = {"/send"}, method = RequestMethod.POST)
    public String sendEmailWithAttachment(Model model,
                             HttpServletRequest request) {

             //Logic to create PDF file

            emailService.sendMessageWithAttachment(to,subject,body, pdfFile);

    }

public interface EmailService{

      sendMessageWithAttachment(
          String to, String subject, String text, String pathToAttachment, FileSystemResource pdfFileToSend); 

}

@Component
public class EmailServiceImpl implements EmailService {

  @Autowired
  private JavaMailSender emailSender;

  @Override
  public void sendMessageWithAttachment(
  String to, String subject, String text, String pathToAttachment, FileSystemResource pdfFileToSend) {
    // ...
    
    MimeMessage message = emailSender.createMimeMessage();
     
    MimeMessageHelper helper = new MimeMessageHelper(message, true);
    
    helper.setFrom("noreply@baeldung.com");
    helper.setTo(to);
    helper.setSubject(subject);
    helper.setText(text);
    
    helper.addAttachment("PDFFile", pdfFileToSend);

    emailSender.send(message);
    // ...
}

Your Javascript then calls this /mail/send endpoint, which should generate the PDF for this user, create an Email, and send Email using spring-boot-mail-starter

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-mail</artifactId>
</dependency>

References:

https://howtodoinjava.com/spring-boot2/send-email-with-attachment/#5