Without this PR:

With this PR:

Also, I'm not sure that my choice of doom-modeline-buffer-minor-mode as an 'inactive' / 'less interesting' face is a good choice, please let me know if there's a better option I should use.

The default pdf modeline looks like this:

It should meet the most of requirements. If it doesn't, you can redefine the modeline, as you did. But I am not convinced yet, since I don't see any major changes here.

The main issue I have with the default modelling is that with long file names the path segment easily pushes the page number off the edge of the modeline (as seen in my screenshot).

IMO this is problematic as the page information is highly desirable, so I moved the segment in front of the buffer info.

Now as you move through the pages as you go from 1 to 2 digits etc. the width changes and the rest of the modeline shifts.
I also consider this behaviour to be undesirable, hence the constant width modification.

Lastly, when viewing a file in pdf-mode I don't think the lock gives any useful information, and you can still modify the PDF itself adding annotations etc. So, I've replaced buffer-info with a buffer-name segment that just gives the name, and added a PDF icon in front.

Does this help clarify the motivation behind these modifications?

Well, doom-modeline-buffer-file-name-style should address the issue of long file names with path. The position of pdf-pages is to keep consistence of the buffer-position in other buffers. Of course you can redefined the modeline as I clarified.

Hmmm, I do see the sense in consistency, I just wonder if it's worth making an exception for paged documents.

Maybe you just need a new modeline for pdf buffers, while not changing so many codes 😄️

(doom-modeline-def-modeline 'my-pdf
  '(bar window-number matches pdf-pages buffer-info-simple)
  '(misc-info major-mode process vcs))

Oh but then you get the cascade of "would be better if this was changed" 😅 (mentioned in my earlier commit) ... leading to what you see here.

FWIW even if I change the file name style to file-name I still see page-info-disappearing issues with some PDFs:

If the buffer/file name is too long, same issue for other files. It also exists in vanilla Emacs.
I am curious why there is no file icon for pdf files in your env. Are you not using pdf-tools?

If the buffer/file name is too long, same issue for other files.

Yea, I'm just of the opinion that you don't tend to care as much about buffer position (in a text file) compared to page number (pdfs).

I am curious why there is no file icon for pdf files in your env. Are you not using pdf-tools?

I just have file-icons off.

Then I suggest to add an option to truncate the file name with while displaying.

This reminds me of #345 😛, regardless I get the impression that with this issue, you'd like to leave things as they currently are and any other layout should be in personal config? If so, I'll close this PR.

yep, I'd like leave it to the private configurations.

Alrighty then, I'll finish the discussion then. As a final note, I would like to re-express my opinion that this PR produces a nicer modelling than the default, but it's not a huge deal 🙂.

For anyone who happens to be come across this and like the look, I've just noticed an omission from my PR:

(doom-modeline-def-modeline 'pdf
    '(bar window-number matches pdf-pages pdf-icon buffer-name)
    '(misc-info major-mode process vcs))

Try (setq doom-modeline-buffer-file-name-style 'auto). Thus the long buffer name will be truncated as fish-like style.