Modeline: Use own variant of PDF modeline

There are a few changes here to improve the appearance of the PDF modeline:
+ Prioritise the page information, move segment in front of buffer name
+ Create (and use) new buffer-name segment, that doesn't include any
  mode or state information
+ Left-pad `doom-modeline--pdf-pages' to ensure that for a given PDF
  it's always the same width
+ Add a `pdf-icon' segment and place it in front of the buffer name

config.org

@@ -1733,6 +1733,55 @@ We then configure the dictionary we're using in [[*Ispell][Ispell]].
 #+begin_src emacs-lisp
 (set-company-backend! 'ess-r-mode '(company-R-args company-R-objects company-dabbrev-code :separate))
 #+end_src
+** Doom modeline
+Very nice and pretty, however I think the PDF modeline could do with tweaking.
+I raised [[https://github.com/seagle0128/doom-modeline/pull/425][an issue]] on this, however the response was basically "put your
+preferences in your personal config, the current default is sensible" --- so
+here we are.
+
+First up I'm going to want a segment for just the buffer file name, and a PDF
+icon. Then we'll redefine two functions used to generate the modeline.
+
+#+begin_src emacs-lisp
+(after! doom-modeline
+  (doom-modeline-def-segment buffer-name
+    "Display the current buffer's name, without any other information."
+    (concat
+     (doom-modeline-spc)
+     (doom-modeline--buffer-name)))
+
+  (doom-modeline-def-segment pdf-icon
+    "PDF icon from all-the-icons."
+    (concat
+     (doom-modeline-spc)
+     (doom-modeline-icon 'octicon "file-pdf" nil nil
+                         :face (if (doom-modeline--active)
+                                   'all-the-icons-red
+                                 'mode-line-inactive)
+                         :v-adjust 0.02)))
+
+  (defun doom-modeline-update-pdf-pages ()
+    "Update PDF pages."
+    (setq doom-modeline--pdf-pages
+          (let ((current-page-str (number-to-string (eval `(pdf-view-current-page))))
+                (total-page-str (number-to-string (pdf-cache-number-of-pages))))
+            (concat
+             (propertize
+              (concat (make-string (- (length total-page-str) (length current-page-str)) ? )
+                      " P" current-page-str)
+              'face 'mode-line)
+             (propertize (concat "/" total-page-str) 'face 'doom-modeline-buffer-minor-mode)))))
+
+  (doom-modeline-def-segment pdf-pages
+    "Display PDF pages."
+    (if (doom-modeline--active) doom-modeline--pdf-pages
+      (propertize doom-modeline--pdf-pages 'face 'mode-line-inactive)))
+
+  (doom-modeline-def-modeline 'pdf
+    '(bar window-number pdf-pages pdf-icon buffer-name)
+    '(misc-info matches major-mode process vcs)))
+#+end_src
+
 ** Elcord
 #+begin_src emacs-lisp
 (setq elcord-use-major-mode-as-main-icon t)