display empty div.pdf-canvas element as a blank page while it is being rendered

services/web/public/stylesheets/app/editor/pdf.less

@@ -23,7 +23,7 @@
 		.full-size;
 		background-color: @gray-lighter;
 		overflow: scroll;
-		canvas {
+		canvas, div.pdf-canvas {
 			background: white;
 			box-shadow: black 0px 0px 10px;
 		}