The standard LaTeX fbox does not start/end exactly at \linewidth (or \textwidth) and the separators and fboxrules have to be subtracted from the width in order to have a visually pleasing box:
\documentclass[]{scrarticle}
\usepackage{lipsum}
\begin{document}
\lipsum[75]
\ \\ % problematic box
\fbox{\parbox{\textwidth}{
\par \addvspace{0.25cm}
\lipsum[66]
\par \addvspace{0.25cm}}}
\ \\ % nice box
\fbox{\parbox{\dimeval{\linewidth-2\fboxsep-2\fboxrule}}{
\par \addvspace{0.25cm}
\lipsum[66]
\par \addvspace{0.25cm}}}
\par \addvspace{0.25cm}
\end{document}