.banner-area-book1 {
    background-image: -moz-linear-gradient(90deg, #0f2027 0%, #203a43 50%, #2c5364 100%);
    background-image: -webkit-linear-gradient(90deg, #0f2027 0%, #203a43 50%, #2c5364 100%);
    background-image: -ms-linear-gradient(90deg, #0f2027 0%, #203a43 50%, #2c5364 100%);
  }
  .banner-area-book2 {
    background-image: -moz-linear-gradient(90deg, #134e5e 0%, #71b280 100%);
    background-image: -webkit-linear-gradient(90deg, #134e5e 0%, #71b280 100%);
    background-image: -ms-linear-gradient(90deg, #134e5e 0%, #71b280 100%);
  }
  .banner-area-book3 {
    background-image: -moz-linear-gradient(90deg, #42275a 0%, #734b6d 100%);
    background-image: -webkit-linear-gradient(90deg, #42275a 0%, #734b6d 100%);
    background-image: -ms-linear-gradient(90deg, #42275a 0%, #734b6d 100%);
  }
  .banner-area-book4 {
    background-image: -moz-linear-gradient(90deg, #141e30 0%, #243b55 100%);
    background-image: -webkit-linear-gradient(90deg, #141e30 0%, #243b55 100%);
    background-image: -ms-linear-gradient(90deg, #141e30 0%, #243b55 100%);
  }
  .banner-area-book5 {
    background-image: -moz-linear-gradient(90deg, #ff512f 0%, #dd2476 100%);
    background-image: -webkit-linear-gradient(90deg, #ff512f 0%, #dd2476 100%);
    background-image: -ms-linear-gradient(90deg, #ff512f 0%, #dd2476 100%);
  }
  .banner-area-book6 {
    background-image: -moz-linear-gradient(90deg, #2c3e50 0%, #4ca1af 100%);
    background-image: -webkit-linear-gradient(90deg, #2c3e50 0%, #4ca1af 100%);
    background-image: -ms-linear-gradient(90deg, #2c3e50 0%, #4ca1af 100%);
  }
  .banner-area-book7 {
    background-image: -moz-linear-gradient(90deg, #654ea3 0%, #eaafc8 100%);
    background-image: -webkit-linear-gradient(90deg, #654ea3 0%, #eaafc8 100%);
    background-image: -ms-linear-gradient(90deg, #654ea3 0%, #eaafc8 100%);
  }
  .pdf-container {
    display: flex;
    justify-content: center; /* canh ngang giữa */
    margin: 0;
    height: 100vh;
  }

  .pdf-frame {
    width: 90%;
    height: 100%;
    border: none;
  }

  @media (max-width: 768px) {
    .pdf-frame {
      width: 100%;   /* full chiều ngang */
      height: 100vh; /* cao toàn màn hình */
    }
  }