Does someone knows how to insert a horizontal line separator in a RTF file?
Example:
some text
some other text
Answer
You could use the \dpline commands (See the section on "Drawing Objects" on page 157 in the RTF Spec 1.9.1).
Here's a sample that should do what you want:
{\rtf1
{\pard some text.\par}
{\pard {\*\do\dobxcolumn\dobypara\dodhgt
\dpline\dpxsize9200\dplinesolid\dplinew30}\par}
{\pard some other text.\par}
}
RTF Output Preview: 
You customize the line color with \dplineco[rgb], width with \dplinew), line-styles etc.
And if you're interested in RTF, you would find the RTF Pocket Guide helpful -- it is much less boring than the RTF Spec! Specifically, it talks about other ways of drawing a horizontal line.