Links in PDF should be opened in new background tabs when ctrl + left clicked.

chrome/browser/resources/pdf/pdf.js

 // Copyright 2013 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.
 
 'use strict';
 
 /**
  * @return {number} Width of a scrollbar in pixels
  */
 function getScrollbarWidth() {
@@ skipping 38 matching lines @@
 function onNavigateInCurrentTab(isInTab, isSourceFileUrl, url) {
   // When the PDFviewer is inside a browser tab, prefer the tabs API because
   // it can navigate from one file:// URL to another.
   if (chrome.tabs && isInTab && isSourceFileUrl)
     chrome.tabs.update({url: url});
   else
     window.location.href = url;
 }
 
 /**
- * Called when navigation happens in the new tab.
- * @param {string} url The url to be opened in the new tab.
+ * Called when navigation happens in the new background tab.
+ * @param {string} url The url to be opened in the new background tab.
  */
-function onNavigateInNewTab(url) {
+function onNavigateInNewBackgroundTab(url) {
   // Prefer the tabs API because it guarantees we can just open a new tab.
   // window.open doesn't have this guarantee.
   if (chrome.tabs)
-    chrome.tabs.create({url: url});
+    chrome.tabs.create({url: url, active: false});
   else
     window.open(url);

[jaepark, 2016/07/14 22:52:39]

I wasn't able to find a way to create a new background tab without using
chrome.tabs. Any ideas? 

[Lei Zhang, 2016/07/14 22:57:52]

I think this is just best effort.

[jaepark, 2016/07/15 00:23:56]

Acknowledged.

 }
 
 /**
  * Whether keydown events should currently be ignored. Events are ignored when
  * an editable element has focus, to allow for proper editing controls.
  * @param {HTMLElement} activeElement The currently selected DOM node.
  * @return {boolean} True if keydown events should be ignored.
  */
 function shouldIgnoreKeyEvents(activeElement) {
   while (activeElement.shadowRoot != null &&
@@ skipping 178 matching lines @@
   document.addEventListener('mouseout', this.handleMouseEvent_.bind(this));
 
   var isInTab = this.browserApi_.getStreamInfo().tabId != -1;
   var isSourceFileUrl =
       this.originalUrl_.indexOf('file://') == 0;
   this.navigator_ = new Navigator(this.originalUrl_,
                                   this.viewport_, this.paramsParser_,
                                   onNavigateInCurrentTab.bind(undefined,
                                                               isInTab,
                                                               isSourceFileUrl),
-                                  onNavigateInNewTab);
+                                  onNavigateInNewBackgroundTab);
   this.viewportScroller_ =
       new ViewportScroller(this.viewport_, this.plugin_, window);
 
   // Request translated strings.
   chrome.resourcesPrivate.getStrings('pdf', this.handleStrings_.bind(this));
 }
 
 PDFViewer.prototype = {
   /**
    * @private
@@ skipping 600 matching lines @@
    * Each bookmark is an Object containing a:
    * - title
    * - page (optional)
    * - array of children (themselves bookmarks)
    * @type {Array} the top-level bookmarks of the PDF.
    */
   get bookmarks() {
     return this.bookmarks_;
   }
 };