Checking if layout (.pdf or .dvi) has changed after deleting LaTeX comments and/or unused macros from the source .tex file

Question

Edit. My question can be reworded as: "How can I check, by command line, if different sources generate exacty the same .pdf layout?"

I'm improving some emacs scripts of mine that I wrote to remove comments and/or unused macros from the source .tex file.

Here's a sample or my code:

(save-buffer)
(call-process-shell-command
 (concat "latex \"\\let\\oldExecuteOptions\\ExecuteOptions\"\\\n"
     "\"\\def\\ExecuteOptions#1{\\oldExecuteOptions{#1,draft}}\"\\\n"
     "\"\\nonstopmode\\input{" (buffer-name) "}\";"
     "mv " (file-name-sans-extension (buffer-name)) ".pdf /tmp/"  ) nil nil)

;;; =========================================================================
;;; =========================== "COMMENT" AMBIENT ===========================
;;; =========================================================================

;;The "comment" ambient is defined in the "verbatim" e "comment packages"
(if (string-match-p "\\\\includecomment[\s\t\n]*{comment}" (buffer-string))
    (read-string "SOME WARNING HERE...")

  ;; *ELSE*
  (progn
    (goto-char (point-min))
    (while (search-forward-regexp "\\\\begin[\s\t\n]*{comment}" nil t)
      (save-excursion
    (let ((b (make-marker))
          (e (make-marker)))
      (set-marker b (match-beginning 0))
      (search-forward-regexp "\\\\end[\s\t\n]*{comment}" nil t)
      (set-marker e (point))
      (goto-char b)
      (unless (nth 4 (syntax-ppss))

        (comment-region b e))
      )))
    (save-buffer)
    (call-process-shell-command
     (concat "latex \"\\let\\oldExecuteOptions\\ExecuteOptions\"\\\n"
         "\"\\def\\ExecuteOptions#1{\\oldExecuteOptions{#1,draft}}\"\\\n"
         "\"\\nonstopmode\\input{" (buffer-name) "}\"" ) nil nil)
    )
  )

As can be seen:

1. I first compile my source by passing the draft option by command line, saving/moving my output in the /tmp/ directory.
2. Then I "comment" (with %) and remove the "comment" ambients from the source .tex file.
3. Finally I compile my cleaned source, again with the draft option...

So I first have:

\documentclass[11pt]{article}
\pdfoutput=1
\usepackage{comment}
\usepackage{blindtext}

\pagestyle{empty}

\begin{document}

\blindtext

\begin{comment}

\blindtext

\end{comment}

\end{document}

and then

\documentclass[11pt]{article}
\pdfoutput=1
\usepackage{comment}
\usepackage{blindtext}

\pagestyle{empty}

\begin{document}

\blindtext

\end{document}

My question is: "How can I check by command line, if the layout has (or not) changed?".

I tried to compare the two pdfs by command line:

cmp mydoc.pdf /tmp/mydoc.pdf

but it always says that the files are different. But I know the layout has unchanged.

If I try with diffpdf (a GUI program) it says "the pdfs appear to be the same" but, as I said, I need a command line solution. I mean I need to get the comparison result in the stdout of my terminal so I can use it in my scripts (e.g. with the emacs' shell-command-to-string fuction). I do not need (and I don't want) to visual check the layout in this stage.

Please, let me know if I've been able to explain my purpose in a clear way.

NOTE. This is a dummy example but in some cases could be tricky to check if some changes will affect the layout.

My considerations. After many attempts using a "file content" approach (I tried a faketime command approach (see here) but it works only with small and simple files) I'm convinced that I need a "layout approach". A tool like diffpdf (with the --appearance option) that could echo in the stdout could be the solution.