[AUCTeX] default TeX-PDF-mode in Auctex

[jlr_0]

Since I'd like all my .tex and .ltx files to open in TeX-PDF mode, this is what
I have placed in my init file.

,----
| (add-hook 'LaTeX-mode-hook
|         '(lambda ()
|            (setq TeX-master 'dwim)
|            (TeX-PDF-mode-on)
|            ))
| (add-hook 'plain-tex-mode-hook
|         'latex-mode)
`----

When opening a new .tex file, I'm asked about the master file and here is the
text that is placed at the end of the file:

,----
| %%% Local Variables: 
| %%% mode: latex
| %%% TeX-master: t
| %%% End: 
`----

This seems ok because when I run `TeX-command-master' (C-c C-c), pdflatex is
automatically called.

However, when opening a new .ltx file (C-x C-f), I'm asked about the master file
but no local variable is ever put at the end of the file.

1. How can I have the local variable show (a) the right mode, as well as the
   (b) `TeX-master" file when opening a .ltx file?

2. Besides, in order to always have TeX/LaTeX file in TeX-PDF mode shouldn't the
   local variables read like this instead?

,----
| %%% Local Variables: 
| %%% mode: TeX-PDF
| %%% TeX-master: t
| %%% End: 
`----

3. In that case, opening such a file leads to toggling TeX-PDF-mode to off and
   the buffer is in plain latex-mode.  How is that?

-- 
jlk