diff --git a/src/display/api.js b/src/display/api.js
index 2c4d4c6d261a4..453e073a5daa6 100644
--- a/src/display/api.js
+++ b/src/display/api.js
@@ -2541,7 +2541,7 @@ class WorkerTransport {
         numPages: this._numPages,
         annotationStorage:
           (annotationStorage && annotationStorage.getAll()) || null,
-        filename: this._fullReader.filename,
+        filename: this._fullReader ? this._fullReader.filename : null,
       })
       .finally(() => {
         annotationStorage.resetModified();
diff --git a/web/app.js b/web/app.js
index a486e02cd0967..6b06e6aed76c4 100644
--- a/web/app.js
+++ b/web/app.js
@@ -874,7 +874,7 @@ const PDFViewerApplication = {
     );
   },
 
-  download() {
+  download({ sourceEventType = "download" }) {
     function downloadByUrl() {
       downloadManager.downloadUrl(url, filename);
     }
@@ -902,12 +902,12 @@ const PDFViewerApplication = {
       .getData()
       .then(function (data) {
         const blob = new Blob([data], { type: "application/pdf" });
-        downloadManager.download(blob, url, filename);
+        downloadManager.download(blob, url, filename, sourceEventType);
       })
       .catch(downloadByUrl); // Error occurred, try downloading with the URL.
   },
 
-  save() {
+  save({ sourceEventType = "download" }) {
     if (this._saveInProgress) {
       return;
     }
@@ -927,7 +927,7 @@ const PDFViewerApplication = {
     // When the PDF document isn't ready, or the PDF file is still downloading,
     // simply download using the URL.
     if (!this.pdfDocument || !this.downloadComplete) {
-      this.download();
+      this.download({ sourceEventType });
       return;
     }
 
@@ -936,10 +936,10 @@ const PDFViewerApplication = {
       .saveDocument(this.pdfDocument.annotationStorage)
       .then(data => {
         const blob = new Blob([data], { type: "application/pdf" });
-        downloadManager.download(blob, url, filename);
+        downloadManager.download(blob, url, filename, sourceEventType);
       })
       .catch(() => {
-        this.download();
+        this.download({ sourceEventType });
       })
       .finally(() => {
         this._saveInProgress = false;
@@ -1722,6 +1722,7 @@ const PDFViewerApplication = {
     eventBus._on("presentationmode", webViewerPresentationMode);
     eventBus._on("print", webViewerPrint);
     eventBus._on("download", webViewerDownload);
+    eventBus._on("save", webViewerSave);
     eventBus._on("firstpage", webViewerFirstPage);
     eventBus._on("lastpage", webViewerLastPage);
     eventBus._on("nextpage", webViewerNextPage);
@@ -1800,6 +1801,7 @@ const PDFViewerApplication = {
     eventBus._off("presentationmode", webViewerPresentationMode);
     eventBus._off("print", webViewerPrint);
     eventBus._off("download", webViewerDownload);
+    eventBus._off("save", webViewerSave);
     eventBus._off("firstpage", webViewerFirstPage);
     eventBus._off("lastpage", webViewerLastPage);
     eventBus._off("nextpage", webViewerNextPage);
@@ -2334,16 +2336,22 @@ function webViewerPresentationMode() {
 function webViewerPrint() {
   window.print();
 }
-function webViewerDownload() {
+function webViewerDownloadOrSave(sourceEventType) {
   if (
     PDFViewerApplication.pdfDocument &&
     PDFViewerApplication.pdfDocument.annotationStorage.size > 0
   ) {
-    PDFViewerApplication.save();
+    PDFViewerApplication.save({ sourceEventType });
   } else {
-    PDFViewerApplication.download();
+    PDFViewerApplication.download({ sourceEventType });
   }
 }
+function webViewerDownload() {
+  webViewerDownloadOrSave("download");
+}
+function webViewerSave() {
+  webViewerDownloadOrSave("save");
+}
 function webViewerFirstPage() {
   if (PDFViewerApplication.pdfDocument) {
     PDFViewerApplication.page = 1;
diff --git a/web/download_manager.js b/web/download_manager.js
index d5b6565f160fc..b5d312681011c 100644
--- a/web/download_manager.js
+++ b/web/download_manager.js
@@ -64,7 +64,13 @@ class DownloadManager {
     download(blobUrl, filename);
   }
 
-  download(blob, url, filename) {
+  /**
+   * @param sourceEventType {string} Used to signal what triggered the download.
+   *   The version of PDF.js integrated with Firefox uses this to to determine
+   *   which dialog to show. "save" triggers "save as" and "download" triggers
+   *   the "open with" dialog.
+   */
+  download(blob, url, filename, sourceEventType = "download") {
     if (navigator.msSaveBlob) {
       // IE10 / IE11
       if (!navigator.msSaveBlob(blob, filename)) {
diff --git a/web/firefoxcom.js b/web/firefoxcom.js
index 985ca053054ac..2e5445b2cc1a4 100644
--- a/web/firefoxcom.js
+++ b/web/firefoxcom.js
@@ -115,7 +115,7 @@ class DownloadManager {
     );
   }
 
-  download(blob, url, filename) {
+  download(blob, url, filename, sourceEventType = "download") {
     const blobUrl = URL.createObjectURL(blob);
     const onResponse = err => {
       if (err && this.onerror) {
@@ -130,6 +130,7 @@ class DownloadManager {
         blobUrl,
         originalUrl: url,
         filename,
+        sourceEventType,
       },
       onResponse
     );
@@ -231,6 +232,17 @@ class MozL10n {
   }
 })();
 
+(function listenSaveEvent() {
+  const handleEvent = function ({ type, detail }) {
+    if (!PDFViewerApplication.initialized) {
+      return;
+    }
+    PDFViewerApplication.eventBus.dispatch(type, { source: window });
+  };
+
+  window.addEventListener("save", handleEvent);
+})();
+
 class FirefoxComDataRangeTransport extends PDFDataRangeTransport {
   requestDataRange(begin, end) {
     FirefoxCom.request("requestDataRange", { begin, end });