diff --git a/evil-doc-view.el b/evil-doc-view.el
index 995de21..f549707 100644
--- a/evil-doc-view.el
+++ b/evil-doc-view.el
@@ -45,12 +45,15 @@
     "G" 'doc-view-last-page
     "J" 'doc-view-goto-page
     (kbd "<return>") 'image-next-line
+
+    ;; zoom
     "+" 'doc-view-enlarge
     "=" 'doc-view-enlarge
     "0" 'doc-view-scale-reset
     "-" 'doc-view-shrink
-    "W" 'doc-view-fit-width-to-window
-    "H" 'doc-view-fit-height-to-window
+
+    "W" 'doc-view-fit-width-to-window ; Like evil-image.
+    "H" 'doc-view-fit-height-to-window ; Like evil-image.
     "P" 'doc-view-fit-page-to-window
     "X" 'doc-view-kill-proc
 
diff --git a/evil-pdf.el b/evil-pdf.el
index 44302b6..4cb1dc6 100644
--- a/evil-pdf.el
+++ b/evil-pdf.el
@@ -73,6 +73,7 @@
     "'" 'pdf-view-jump-to-register
     "m" 'pdf-view-position-to-register
 
+    ;; zoom
     "+" 'pdf-view-enlarge
     "-" 'pdf-view-shrink
     "0" 'pdf-view-scale-reset
diff --git a/readme.org b/readme.org
index ea8f1d9..935da8c 100644
--- a/readme.org
+++ b/readme.org
@@ -294,7 +294,13 @@ If not, it can be used to display help.
 
 ?
 
+*** Zooming (~+~, ~-~, ~=~, ~0~)
 
+- ~+~ and ~-~ have obvious meanings.
+
+- ~0~ has a somewhat intuitive meaning, plus it is next to ~+~ and ~-~ on QWERTY.
+
+- ~=~ is useful as a synonym for ~+~ because it is the unshifted key of ~+~ on QWERTY.
 
 ** Modes left behind