How to make page numbering with ligature roman style?

Question

I use roman page number for front matter, I was just wondering, is it possible to make these roman numbers as ligature?

See.

Roman:

and

roman:

Answer 1

This introduces two new numberings: xroman and xRoman, that try to do what you want. We had to introduce our own for-cycle, because we have to make everything expandable. In the result, we just add a negative space between the letters. We use small-caps for the lower-case variant, and we use a trick with a robust command to add the dot on i.

EDIT: The new version works with hyperref package without causing warnings.

EDIT 2: The new version allows horizontal adjustments of the dot over i.

\documentclass{article}

\pagenumbering{xroman}

\usepackage{hyperref}

\makeatletter
\DeclareRobustCommand \i@xroman{\rlap{\kern-0.007em\.{}}i}
\def \@xroman@for #1#2\@nil{\if i#1\i@xroman\else#1\fi\ifx\relax#2\relax\else\hskip-0.093em\@xroman@for#2\@nil\fi}
\def \@xRoman@for #1#2\@nil{\uppercase{#1}\ifx\relax#2\else\hskip-0.09em\@xRoman@for#2\@nil\fi}
\newcommand \@Xroman [1]{\begingroup\scshape\expandafter\expandafter\expandafter\@xroman@for\@roman{#1}\relax\@nil\endgroup}
\newcommand \@XRoman [1]{\begingroup        \expandafter\expandafter\expandafter\@xRoman@for\@roman{#1}\relax\@nil\endgroup}
\newcommand \xroman [1]{\expandafter\@xroman\csname c@#1\endcsname}
\newcommand \xRoman [1]{\expandafter\@xRoman\csname c@#1\endcsname}
\AtBeginDocument{\@ifundefined{texorpdfstring}{
  \let\@xroman\@Xroman
  \let\@xRoman\@XRoman
}{
  \newcommand \@xroman [1]{\texorpdfstring{\@Xroman{#1}}{\@roman{#1}}}
  \newcommand \@xRoman [1]{\texorpdfstring{\@XRoman{#1}}{\@Roman{#1}}}
}}
\makeatother  

\begin{document}

\setcounter{page}{3678}

\section{ABC}\label{abc}

This is Section \ref{abc} on page \pageref{abc}.

hello

\end{document}

Answer 2

There is another minor issue, when using \pagenumbering{xroman} or {xRoman}, hyperref warning info:

Package hyperref Warning: Token not allowed in a PDF string (PDFDocEncoding): (hyperref) removing `\uppercase' on input line 53.

Package hyperref Warning: Token not allowed in a PDF string (PDFDocEncoding): (hyperref) \kern -1.07996pt (hyperref) removed on input line 53.

Package hyperref Warning: Token not allowed in a PDF string (PDFDocEncoding): (hyperref) removing `\uppercase' on input line 53.

Package hyperref Warning: Token not allowed in a PDF string (PDFDocEncoding): (hyperref) removing `\endgroup' on input line 53.

I know this is like using Greek symbol in Hyperref, and hence the warning inform arose.

The dot of lower case roman number i is a bit right aligned compared to original i, see [Libertine font] The upper case is ok then.