Ensure PDF integration is loaded in PDF documents

The recent refactor to change the relationship between the `Guest` and `Sidebar`
classes broke PDF.js integration because the `PdfSidebar` class is no
longer able to modify the config that is passed to the `Guest` class.
Previously it would pass `PDF: {}` in the config to the `super()` call
which would then result in the `Guest` class loading the PDF
integration.

This now has to be done in the `index.js` module. We already had to do
this previously for ebooks. Now it has to be done for all scenarios.

src/annotator/index.js

@@ -52,10 +52,6 @@ function init() {
   let SidebarClass = isPDF ? PdfSidebar : Sidebar;
 
   if (config.subFrameIdentifier) {
-    // Make sure the PDF plugin is loaded if the subframe contains the PDF.js viewer.
-    if (isPDF) {
-      config.PDF = {};
-    }
     SidebarClass = null;
 
     // Other modules use this to detect if this
@@ -66,6 +62,11 @@ function init() {
 
   config.pluginClasses = pluginClasses;
 
+  // Load the PDF anchoring/metadata integration.
+  if (isPDF) {
+    config.PDF = {};
+  }
+
   const guest = new Guest(document.body, config);
   const sidebar = SidebarClass
     ? new SidebarClass(document.body, guest, config)

src/annotator/pdf-sidebar.js

@@ -7,7 +7,6 @@ import Sidebar from './sidebar';
  */
 
 const defaultConfig = {
-  PDF: {},
   BucketBar: {
     scrollables: ['#viewerContainer'],
   },