@PostMapping("/read")
    public String readBook(Model model) throws IOException {
        
        Model.addAtrribute("pdf",bookRepo.findById(id).get());
      
        return "redirect:/book";
    }

 @PostMapping
    public String addingBook(@PathParam("name")String name,
                             @PathParam("url")String url) throws IOException {
        Book book=new Book();
        File file = new File(url);
        FileInputStream fis = new FileInputStream(file);
        ByteArrayOutputStream bos = new ByteArrayOutputStream();
        byte[] buf = new byte[100000];
        try {
            for (int readNum; (readNum = fis.read(buf)) != -1;) {
                bos.write(buf, 0, readNum);
            }
        } catch (IOException ex) {
            System.out.println("ERROR!");
        }
        byte[] bytes = bos.toByteArray();
        book.setName(name);
        book.setPdf(bytes);
        bookRepo.save(book);
        return "redirect:/book";
    }

 @PostMapping("/read")
    public String readBook(Model model) throws IOException {
        model.addAttribute("pdf",bookRepo.findById(15L));
        return "book";
    }

<#if pdf??>
<object data="data:application/pdf,<${pdf}>" width="800" height="900" type="application/pdf" ></object>
</#if>

Request header is too large

  @GetMapping("/read")
    public String readBook(Model model) throws IOException {
//        File file = new File("C:\\Users\\stanislav.alekseev\\IdeaProjects\\sdo\\src\\main\\resources\\static\\book.pdf");
//        Book b = bookRepo.findById(16L).get();
//        FileOutputStream fo = new FileOutputStream(file);
//        fo.write(b.getPdf());
        File file = new File("C:\\Users\\stanislav.alekseev\\Desktop\\pdf\\2.pdf");
        model.addAttribute("pdf",file);
        return "book";
    }

<object data="${pdf}" width="800" height="900" type="application/pdf" ></object>