Unwanted offset for horizontal centering

Question

I want to create a blank page (no header, footer and page numbering) with only one word centered horizontally and 35% vertically from the top, with page margins in mind. For my headers/footers I use the package fancyhdr, and the document is compiled by pdflatex.

However, if I use the following code for my individual *.tex file I notice that the horizontal alignment is off by a couple of millimeters to the left. What is the cause of this?

\begingroup
    \thispagestyle{empty}
    \centering
    \vspace*{.34\textheight}
    \fbox{{\normalfont\fontsize{25}{25}\bfseries Anhang}}
    \vspace*{\fill}

\endgroup

Preview:

Answer 1

The reason of this is that the line breaking after \fbox{...} generates a space! Write % after \fbox: \fbox{...}%

Answer 2

Replace \centering with the center environment (maybe you'll have to adapt the vertical spacing):

\documentclass{article}
\usepackage[utf8]{inputenc} 
\usepackage[T1]{fontenc} 
\usepackage[showframe]{geometry} 

\begin{document}

\begingroup
    \thispagestyle{empty}
    \vspace*{.34\textheight}
\begin{center}
    \fbox{{\normalfont\fontsize{25}{25}\bfseries Anhang}}
\end{center}
    \vspace*{\fill}

\endgroup

\end{document} 

Answer 3

It's best to use \vspace between paragraphs. I also set the word in a zero height and depth box, after moving it down by half its total height plus depth.

\documentclass{book}

\usepackage{showframe}% for seeing the page margins

\begin{document}

% whatever before the appendix

\cleardoublepage

\begingroup
\thispagestyle{empty}
\centering

\fboxsep=-\fboxrule % just for the example

\vspace*{\stretch{0.35}}

\noindent\raisebox{-0.5\totalheight}[0pt][0pt]{%
%  \normalfont\fontsize{25}{0}\bfseries Anhang% <--- should be this
  \normalfont\fontsize{25}{0}\bfseries \fbox{Anhang}% <--- for the measuring
}

\vspace*{\stretch{0.65}}

\endgroup

\cleardoublepage

\end{document}