> On Dec 16, 2017, at 8:48 AM, Vicente Vera <vicentemvp@gmail.com> wrote:
>
> I'm trying to export a simple document to a LaTeX buffer, but just the
> main content, without a preamble nor the "document" environment.

You want `(org-latex-export-as-latex nil nil nil t)' or `C-c C-e C-b l L'.


,----[ C-h f org-latex-export-as-latex RET ]
| org-latex-export-as-latex is an interactive autoloaded compiled Lisp
| function in ‘ox-latex.el’.
|
| (org-latex-export-as-latex &optional ASYNC SUBTREEP VISIBLE-ONLY
| BODY-ONLY EXT-PLIST)
|
| Export current buffer as a LaTeX buffer.
|
| [...]
|
| When optional argument BODY-ONLY is non-nil, only write code
| between "\begin{document}" and "\end{document}".
|
| [...]
`----

> The main objective is to export several Org documents which are parts of a
> larger LaTeX report. These parts are then included by means of the
> "input" command.

Why not use include keywords in the org document?  They can, if needed, refer to sections in the parent document.

See (info "(org) Include files")

HTH,

Chuck