Use a style tag with an ID instead of keeping a reference to it.

src/fonts.js

@@ -20,9 +20,6 @@ var kPDFGlyphSpaceUnits = 1000;
 // Until hinting is fully supported this constant can be used
 var kHintingEnabled = false;
 
-// A reference to a reusable style sheet.
-var styleSheet;
-
 var FontFlags = {
   FixedPitch: 1,
   Serif: 2,
@@ -2377,14 +2374,14 @@ var Font = (function FontClosure() {
                  window.btoa(data) + ');');
       var rule = "@font-face { font-family:'" + fontName + "';src:" + url + '}';
 
-
-      if(!styleSheet) {
-        var styleElement = document.createElement('style');
-        document.documentElement.getElementsByTagName('head')[0].appendChild(
-          styleElement);
-
-        styleSheet = styleElement.sheet;
+      var styleElement = document.getElementById('PDFJS_FONT_STYLE_TAG');
+      if (!styleElement) {
+          styleElement = document.createElement('style');
+          document.documentElement.getElementsByTagName('head')[0].appendChild(
+            styleElement);   
       }
+
+      var styleSheet = styleElement.sheet;
       styleSheet.insertRule(rule, styleSheet.cssRules.length);
 
       if (PDFJS.pdfBug && FontInspector.enabled)