Use header button classes in the PDF button.

services/web/app/views/project/editor/header.pug

@@ -15,7 +15,7 @@ header.toolbar.toolbar-header.toolbar-with-labels(
 			i.fa.fa-fw.fa-level-up
 
 	span(ng-controller="PdfViewToggleController")
-		a(
+		a.btn.btn-full-height.btn-full-height-no-border(
 			href,
 			ng-show="ui.pdfLayout == 'flat' && fileTreeClosed",
 			tooltip="PDF",

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

@@ -65,6 +65,10 @@
 			right: 4px;
 		}
 	}
+	.btn-full-height-no-border {
+		border-right: 0;
+		border-left: 0;
+	}
 
 	.toolbar-left {
 		float: left;