The extension should urlencode the pdf when sending it over to the viewer.

extensions/firefox/components/pdfContentHandler.js

@@ -56,7 +56,8 @@ pdfContentHandler.prototype = {
       throw NS_ERROR_WONT_HANDLE_CONTENT;
 
     aRequest.cancel(Cr.NS_BINDING_ABORTED);
-    window.location = url.replace('%s', targetUrl);
+    aRequest.cancel(Cr.NS_BINDING_ABORTED);
+    window.location = url.replace('%s', encodeURIComponent(targetUrl));
   },
 
   classID: Components.ID('{2278dfd0-b75c-11e0-8257-1ba3d93c9f1a}'),