How do I set my page numbers to the same size through the whole document?

Question

\documentclass{book}

\usepackage{kantlipsum}
\usepackage{appendix}

\usepackage{fontspec} % compile w/XeLaTeX
\setmainfont{Perpetua}

\usepackage{fancyhdr}
\pagestyle{fancy}
\fancyhf{} % clear all header and footer fields
\renewcommand{\headrule}{} % Remove the horizontal line from the header
\fancyfoot[C]{\fontfamily{ppl}\fontsize{11}{13}\selectfont\thepage} % set the footer centred and using Perpetua 11 font

\newcommand{\appendixchapter}{% https://tex.stackexchange.com/a/681868/269662
  \cleardoublepage
  \fancyfoot[C]{\fontfamily{ppl}\fontsize{11}{13}\selectfont\thepage}
  \pagenumbering{arabic}
  \setcounter{page}{1}%
  \renewcommand{\thepage}{\thechapter\arabic{page}}%
  \chapter
}

\begin{document}
  \fontsize{13pt}{15.6pt}\selectfont
  \chapter{A chapter}
  \kant[1]
  \appendix
    \appendixchapter{An appendix}
    \kant
    \appendixchapter{Another appendix}
    \kant
\end{document}

The page numbers are set to Perpetua 11, which compiles like that. However, it only appears to do so on the first page of the (appendix) chapter. Then, the numbers get bigger again. How do I set the page numbering to Perpetua 11 throughout the whole document?

Answer 1

You shouldn't be using \fontsize{13.5}{15.6}\selectfont. Instead use fontsize.

With \usepackage[fontsize=13.5pt]{fontsize}, the baseline skip would be set to 16.19995pt, so it's easy to fix \baselinestretch in order to obtain 15.6pt: just divide to find that

15.6/16.19995 = 0.96296593508004654335

and indeed issuing

\linespread{0.96297}

produces \baselineskip=15.6pt. I'd also set \topskip to the font size.

This said, your code has mysterious \fontfamily{ppl} instructions that are to be removed.

Now the text height is set to 550pt, but

(550-13.5)/15.6 = 34.39102564102564102564

so you don't have an integral number of lines in your pages. You need to reduce a bit the text height:

13.5+34*15.6 = 543.9

But roundings accumulate, so it's better to use 543.91pt.

\documentclass{book}
\usepackage[fontsize=13.5pt]{fontsize}
\usepackage{fontspec} % compile w/XeLaTeX
\usepackage{appendix}

\usepackage{kantlipsum}
\usepackage{fancyhdr}

\setmainfont{Perpetua}[
  Path=/Applications/Microsoft Word.app/Contents/Resources/DFonts/,
  BoldFont=* Bold,
  ItalicFont=* Italic,
  BoldItalicFont=* Bold Italic,
]

\pagestyle{fancy}
\fancyhf{} % clear all header and footer fields
\renewcommand{\headrulewidth}{0pt} % Remove the horizontal line from the header

\fancyfoot[C]{%
  \fontsize{11}{13}\selectfont\thepage
} % set the footer centred and using Perpetua 11 font
% fix also the plain page style
\fancypagestyle{plain}{%
  \fancyhf{}%
  \renewcommand{\headrulewidth}{0pt}%
  \fancyfoot[C]{\fontsize{11}{13}\selectfont\thepage}%
}

\newcommand{\appendixchapter}{% https://tex.stackexchange.com/a/681868/269662
  \cleardoublepage
  \pagenumbering{arabic}%
  \renewcommand{\thepage}{\thechapter\arabic{page}}%
  \chapter
}

% final setting: fix the baseline skip
\linespread{0.96297}
\setlength{\topskip}{13.5pt}
\setlength{\textheight}{543.91pt}

\begin{document}

\chapter{A chapter}

\kant[1]

\appendix

\appendixchapter{An appendix}

\kant

\appendixchapter{Another appendix}

\kant

\end{document}