diff --git a/web/viewer.js b/web/viewer.js
index 87de9fa88..934f8e58c 100644
--- a/web/viewer.js
+++ b/web/viewer.js
@@ -3117,6 +3117,15 @@ window.addEventListener('keydown', function keydown(evt) {
 
   if (cmd === 4) { // shift-key
     switch (evt.keyCode) {
+      case 32: // spacebar
+        if (!PDFView.isPresentationMode &&
+            PDFView.currentScaleValue !== 'page-fit') {
+          break;
+        }
+        PDFView.page--;
+        handled = true;
+        break;
+
       case 82: // 'r'
         PDFView.rotatePages(-90);
         break;