two identical .tex files; one compiles, one does not

[Heiko Oberdiek]

Hello,

On 2022-11-21 18:19, Christopher W. Ryan via texhax wrote:
> For both files, I just used the "pdf" toolbar button in emacs. I used it
> the same way for both files. I believe it is set to execute pdflatex. I
> have emacs 26.2 from Vince Goulet, that has AUCTEX and ESS included.

The TeX compilers (tex, pdftex, luatex, xetex, ...) generate a .log file
that tells in the first line the TeX compiler name and the used format, 
e.g.:

   This is pdfTeX, Version 3.141592653-2.6-1.40.24 (TeX Live 
2022/Cygwin) (preloaded format=pdflatex 2022.4.27)

"\documentclass" requires a LaTeX-based format.

Yours sincerely
   Heiko