.pdf-gallery {
    margin: 5px;
    float: left;
    width: 310px;
    height: 420px;
    position: relative;
    padding:10px 2px 5px 2px;
}

    .pdf-gallery a {
    }

        .pdf-gallery a img {
            width: 100%;
            height: 100%;
            opacity: 1;
            display: block;
            transition: .5s ease;
            backface-visibility: hidden;
        }

.middle {
    transition: .5s ease;
    opacity: 0;
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
    -ms-transform: translate(-50%, -50%);
}

.pdf-gallery:hover img {
    opacity: 0.3;
}

.pdf-gallery:hover .middle {
    opacity: 1;
}

.text {
    background-color: #000;
    color: white;
    font-size: 16px;
    padding: 16px 32px;
    border-radius: 12px;
}