Display:none arrows in disabled state

common/static/js/pdfviewer.js

@@ -202,15 +202,15 @@ PDFJS.disableWorker = true;
             $("#pageNumber").val(pageNum);
 
             // Enable/disable the previous/next buttons
-            if (pageNum > 1) {
-              $("#previous").addClass("enabled");
+            if (pageNum <= 1) {
+              $("#previous").addClass("is-disabled");
             } else {
-              $("#previous").removeClass("enabled");
+              $("#previous").removeClass("is-disabled");
             }
-            if (pageNum < pdfDocument.numPages) {
-              $("#next").addClass("enabled");
+            if (pageNum >= pdfDocument.numPages) {
+              $("#next").addClass("is-disabled");
             } else {
-              $("#next").removeClass("enabled");
+              $("#next").removeClass("is-disabled");
             }
         }
 

lms/static/sass/course/_textbook.scss

@@ -160,13 +160,16 @@ div.book-wrapper {
             @include transition(none);
             vertical-align: middle;
             width: 100%;
-          }
 
-          a.enabled:hover {
-            opacity: 1.0;
-            filter: alpha(opacity=100);
-          }
+            &:hover {
+              opacity: 1.0;
+            }
 
+            &.is-disabled {
+              display:none;
+            }
+          }
+          
           &.last {
             left: 0;