--- a/browser/extensions/pdfjs/README.mozilla
+++ b/browser/extensions/pdfjs/README.mozilla
@@ -1,5 +1,5 @@
 This is the PDF.js project output, https://github.com/mozilla/pdf.js
 
-Current extension version is: 2.4.349
+Current extension version is: 2.4.375
 
-Taken from upstream commit: dced0a38
+Taken from upstream commit: e2b30e9e

--- a/browser/extensions/pdfjs/content/build/pdf.js
+++ b/browser/extensions/pdfjs/content/build/pdf.js
@@ -118,18 +118,18 @@ return /******/ (function(modules) { // 
 /************************************************************************/
 /******/ ([
 /* 0 */
 /***/ (function(module, exports, __w_pdfjs_require__) {
 
 "use strict";
 
 
-var pdfjsVersion = '2.4.349';
-var pdfjsBuild = 'dced0a38';
+var pdfjsVersion = '2.4.375';
+var pdfjsBuild = 'e2b30e9e';
 
 var pdfjsSharedUtil = __w_pdfjs_require__(1);
 
 var pdfjsDisplayAPI = __w_pdfjs_require__(3);
 
 var pdfjsDisplayTextLayer = __w_pdfjs_require__(16);
 
 var pdfjsDisplayAnnotationLayer = __w_pdfjs_require__(17);
@@ -1200,17 +1200,17 @@ function _fetchDocument(worker, source, 
   if (pdfDataRangeTransport) {
     source.length = pdfDataRangeTransport.length;
     source.initialData = pdfDataRangeTransport.initialData;
     source.progressiveDone = pdfDataRangeTransport.progressiveDone;
   }
 
   return worker.messageHandler.sendWithPromise("GetDocRequest", {
     docId,
-    apiVersion: '2.4.349',
+    apiVersion: '2.4.375',
     source: {
       data: source.data,
       url: source.url,
       password: source.password,
       disableAutoFetch: source.disableAutoFetch,
       rangeChunkSize: source.rangeChunkSize,
       length: source.length
     },
@@ -1946,19 +1946,17 @@ class LoopbackPort {
         return cloned.get(value);
       }
 
       let buffer, result;
 
       if ((buffer = value.buffer) && (0, _util.isArrayBuffer)(buffer)) {
         const transferable = transfers && transfers.includes(buffer);
 
-        if (value === buffer) {
-          result = value;
-        } else if (transferable) {
+        if (transferable) {
           result = new value.constructor(buffer, value.byteOffset, value.byteLength);
         } else {
           result = new value.constructor(value);
         }
 
         cloned.set(value, result);
         return result;
       }
@@ -3147,19 +3145,19 @@ const InternalRenderTask = function Inte
       }
     }
 
   }
 
   return InternalRenderTask;
 }();
 
-const version = '2.4.349';
+const version = '2.4.375';
 exports.version = version;
-const build = 'dced0a38';
+const build = 'e2b30e9e';
 exports.build = build;
 
 /***/ }),
 /* 4 */
 /***/ (function(module, exports, __w_pdfjs_require__) {
 
 "use strict";
 
@@ -6997,25 +6995,25 @@ var _util = __w_pdfjs_require__(1);
 var _xml_parser = __w_pdfjs_require__(13);
 
 class Metadata {
   constructor(data) {
     (0, _util.assert)(typeof data === "string", "Metadata: input is not a string");
     data = this._repair(data);
     const parser = new _xml_parser.SimpleXMLParser();
     const xmlDocument = parser.parseFromString(data);
-    this._metadata = Object.create(null);
+    this._metadataMap = new Map();
 
     if (xmlDocument) {
       this._parse(xmlDocument);
     }
   }
 
   _repair(data) {
-    return data.replace(/^([^<]+)/, "").replace(/>\\376\\377([^<]+)/g, function (all, codes) {
+    return data.replace(/^[^<]+/, "").replace(/>\\376\\377([^<]+)/g, function (all, codes) {
       const bytes = codes.replace(/\\([0-3])([0-7])([0-7])/g, function (code, d1, d2, d3) {
         return String.fromCharCode(d1 * 64 + d2 * 8 + d3 * 1);
       }).replace(/&(amp|apos|gt|lt|quot);/g, function (str, name) {
         switch (name) {
           case "amp":
             return "&";
 
           case "apos":
@@ -7074,33 +7072,39 @@ class Metadata {
       if (desc.nodeName.toLowerCase() !== "rdf:description") {
         continue;
       }
 
       for (let j = 0, jj = desc.childNodes.length; j < jj; j++) {
         if (desc.childNodes[j].nodeName.toLowerCase() !== "#text") {
           const entry = desc.childNodes[j];
           const name = entry.nodeName.toLowerCase();
-          this._metadata[name] = entry.textContent.trim();
+
+          this._metadataMap.set(name, entry.textContent.trim());
         }
       }
     }
   }
 
   get(name) {
-    const data = this._metadata[name];
-    return typeof data !== "undefined" ? data : null;
+    return this._metadataMap.has(name) ? this._metadataMap.get(name) : null;
   }
 
   getAll() {
-    return this._metadata;
+    const obj = Object.create(null);
+
+    for (const [key, value] of this._metadataMap) {
+      obj[key] = value;
+    }
+
+    return obj;
   }
 
   has(name) {
-    return typeof this._metadata[name] !== "undefined";
+    return this._metadataMap.has(name);
   }
 
 }
 
 exports.Metadata = Metadata;
 
 /***/ }),
 /* 13 */
@@ -8945,17 +8949,16 @@ var renderTextLayer = function renderTex
         return;
       }
 
       if (this._bounds !== null) {
         expand(this);
         this._bounds = null;
       }
 
-      const NO_PADDING = "0 0 0 0";
       const transformBuf = [],
             paddingBuf = [];
 
       for (var i = 0, ii = this._textDivs.length; i < ii; i++) {
         const div = this._textDivs[i];
 
         const divProps = this._textDivProperties.get(div);
 
@@ -8992,21 +8995,17 @@ var renderTextLayer = function renderTex
 
           if (divProps.paddingLeft > 0) {
             paddingBuf.push(`${divProps.paddingLeft / divProps.scale}px`);
             transformBuf.push(`translateX(${-divProps.paddingLeft / divProps.scale}px)`);
           } else {
             paddingBuf.push(0);
           }
 
-          const padding = paddingBuf.join(" ");
-
-          if (padding !== NO_PADDING) {
-            div.style.padding = padding;
-          }
+          div.style.padding = paddingBuf.join(" ");
 
           if (transformBuf.length) {
             div.style.transform = transformBuf.join(" ");
           }
         } else {
           div.style.padding = null;
           div.style.transform = divProps.originalTransform;
         }

--- a/browser/extensions/pdfjs/content/build/pdf.worker.js
+++ b/browser/extensions/pdfjs/content/build/pdf.worker.js
@@ -118,18 +118,18 @@ return /******/ (function(modules) { // 
 /************************************************************************/
 /******/ ([
 /* 0 */
 /***/ (function(module, exports, __w_pdfjs_require__) {
 
 "use strict";
 
 
-const pdfjsVersion = '2.4.349';
-const pdfjsBuild = 'dced0a38';
+const pdfjsVersion = '2.4.375';
+const pdfjsBuild = 'e2b30e9e';
 
 const pdfjsCoreWorker = __w_pdfjs_require__(1);
 
 exports.WorkerMessageHandler = pdfjsCoreWorker.WorkerMessageHandler;
 
 /***/ }),
 /* 1 */
 /***/ (function(module, exports, __w_pdfjs_require__) {
@@ -218,17 +218,17 @@ var WorkerMessageHandler = {
 
   createDocumentHandler(docParams, port) {
     var pdfManager;
     var terminated = false;
     var cancelXHRs = null;
     var WorkerTasks = [];
     const verbosity = (0, _util.getVerbosityLevel)();
     const apiVersion = docParams.apiVersion;
-    const workerVersion = '2.4.349';
+    const workerVersion = '2.4.375';
 
     if (apiVersion !== workerVersion) {
       throw new Error(`The API version "${apiVersion}" does not match ` + `the Worker version "${workerVersion}".`);
     }
 
     var docId = docParams.docId;
     var docBaseUrl = docParams.docBaseUrl;
     var workerHandlerName = docParams.docId + "_worker";

--- a/browser/extensions/pdfjs/content/web/viewer.js
+++ b/browser/extensions/pdfjs/content/web/viewer.js
@@ -372,17 +372,17 @@ class DefaultExternalServices {
     });
   }
 
 }
 
 exports.DefaultExternalServices = DefaultExternalServices;
 const PDFViewerApplication = {
   initialBookmark: document.location.hash.substring(1),
-  initialized: false,
+  _initializedCapability: (0, _pdfjsLib.createPromiseCapability)(),
   fellback: false,
   appConfig: null,
   pdfDocument: null,
   pdfLoadingTask: null,
   printService: null,
   pdfViewer: null,
   pdfThumbnailViewer: null,
   pdfRenderingQueue: null,
@@ -427,17 +427,18 @@ const PDFViewerApplication = {
     this.bindEvents();
     this.bindWindowEvents();
     const appContainer = appConfig.appContainer || document.documentElement;
     this.l10n.translate(appContainer).then(() => {
       this.eventBus.dispatch("localized", {
         source: this
       });
     });
-    this.initialized = true;
+
+    this._initializedCapability.resolve();
   },
 
   async _readPreferences() {
     if (_app_options.AppOptions.get("disablePreferences") === true) {
       return;
     }
 
     try {
@@ -634,16 +635,24 @@ const PDFViewerApplication = {
     this.pdfSidebar.onToggled = this.forceRendering.bind(this);
     this.pdfSidebarResizer = new _pdf_sidebar_resizer.PDFSidebarResizer(appConfig.sidebarResizer, eventBus, this.l10n);
   },
 
   run(config) {
     this.initialize(config).then(webViewerInitialized);
   },
 
+  get initialized() {
+    return this._initializedCapability.settled;
+  },
+
+  get initializedPromise() {
+    return this._initializedCapability.promise;
+  },
+
   zoomIn(ticks) {
     if (this.pdfViewer.isInPresentationMode) {
       return;
     }
 
     let newScale = this.pdfViewer.currentScale;
 
     do {

--- a/browser/extensions/pdfjs/moz.yaml
+++ b/browser/extensions/pdfjs/moz.yaml
@@ -15,15 +15,15 @@ origin:
   description: Portable Document Format (PDF) viewer that is built with HTML5
 
   # Full URL for the package's homepage/etc
   # Usually different from repository url
   url: https://github.com/mozilla/pdf.js
 
   # Human-readable identifier for this version/release
   # Generally "version NNN", "tag SSS", "bookmark SSS"
-  release: version 2.4.349
+  release: version 2.4.375
 
   # The package's license, where possible using the mnemonic from
   # https://spdx.org/licenses/
   # Multiple licenses can be specified (as a YAML list)
   # A "LICENSE" file must exist containing the full license text
   license: Apache-2.0