Putting a bar in the margin
The framed
package provides a leftbar
environment that does something like what you want:
\documentclass{article}
\usepackage{framed,lipsum}
\begin{document}
\lipsum[1]
\begin{leftbar}
This is a test.
\end{leftbar}
\lipsum[2]
\end{document}
Have a look at changebar
package: https://ctan.org/pkg/changebar
An example:
\documentclass{article}
\usepackage[color]{changebar}
\cbcolor{blue}
\usepackage{lipsum}% for random text only
\begin{document}
\cbstart
\lipsum[1]
\cbend
\lipsum[2]
\end{document}
You can also use PDF annotations with the pdfcomment package
\documentclass{scrartcl}
\usepackage[english]{babel}
\usepackage[latin1]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage{lipsum}
\usepackage[svgnames,rgb]{xcolor}
\usepackage[subject={Top1},author={Josef}]{pdfcomment}
\begin{document}
\begin{pdfsidelinecomment}[color=red,icolor=yellow,caption=inline,linebegin={/Butt},lineend={/Square},linewidth=2bp,linesep=1cm]{Example}
\lipsum[2]
\end{pdfsidelinecomment}
\end{document}