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This question already has an answer here:

Image

I wish to achieve following. There may be many variant of this.

marked as duplicate by AlexG, Jesse, Claudio Fiandrino, Guido, user13907 Aug 28 '14 at 12:24

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If PDF annotations are okay for you:

\documentclass{article}
\usepackage{pdfcomment}
\newcommand*\callout[2]%
{%
  \pdfmarkupcomment[color={ 0.3 0.3 0.3 },opacity=0]{\textbf{#1}}{#2}%
}%
\begin{document}
\callout{If}{If shows the cause ...} it can be detected ...
\end{document}

example

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