Adobe Acrobat has the feature that enables the user to markup words or lines and to add an annotation to these markups. These features do not only exits in Acrobat.
The markup looks like this:

Is it possible to create with LaTeX/LuaLaTeX/XeLaTeX a pdf that already has these markups (or even the annotations)?
To be honest I am not using annotation that much, but I use the marking of lines quite often and it is quite pedestrian to produce a pdf with LaTeX, to open it with Acrobat, to mark the relevant lines and then sadly to recognise an error, create the pdf again and have to markup again...

pdflatexsolution is also sufficient: some time ago, I wrote thepdfmarginparpackage http://mirrors.ctan.org/macros/latex/contrib/pdfmarginpar/doc/pdfmarginpar.pdf to get access to such annotation features. Might be superseded by pdfcomment as proposed by jfbu – Christian Feuersänger Jun 21 '13 at 21:47