joplin/packages/pdf-viewer/index.html

<div id="pdf-root">
    <div id="splash-screen"></div>
</div>
<style>
    #pdf-root {
        width: 100%;
        height: 100%;
    }
    #splash-screen{
        display:flex;
        justify-content:center;
        align-items:center;
        width:100%;
        height:100%;
    }
</style>
<script src="./main.js"></script>