PDF: Tweak font-size and colors to more closely match other MD WebUI

chrome/browser/resources/pdf/index.css

 /* Copyright 2015 The Chromium Authors. All rights reserved.
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file. */
 
 body {
   background-color: rgb(82, 86, 89);
-  font-family: 'Roboto', 'Noto', sans-serif;
+  color: var(--primary-text-color);
+  line-height: 154%;
   margin: 0;
 }
 
 viewer-page-indicator {
   visibility: hidden;
   z-index: 2;
 }
 
 viewer-pdf-toolbar {
   position: fixed;
@@ skipping 23 matching lines @@
   viewer-zoom-toolbar {
     display: none;
   }
 }
 
 @media(max-width: 300px) {
   viewer-zoom-toolbar {
     display: none;
   }
 }