chrome/browser/resources/pdf/pdf_viewer_shared_style.html

<template>
  <style>
    #content {
      height: 100%;
      position: fixed;
      width: 100%;
      z-index: 1;
    }

    #plugin {
      height: 100%;
      position: absolute;
      width: 100%;
    }

    #sizer {
      position: absolute;
      z-index: 0;
    }
  </style>
</template>