Ensure that the PDF viewer has an isolated stacking context (#24003)

GitOrigin-RevId: 2cdaffa8c4387e173f3256864ec15d785931caf2

services/web/frontend/stylesheets/bootstrap-5/pages/editor/pdf.scss

@@ -174,6 +174,8 @@
 }
 
 .pdf-viewer {
+  isolation: isolate;
+
   iframe {
     width: 100%;
     height: 100%;