Hyperref Warnings

Question

Overleaf tells me this:

Why? Thank you so much

First Edit: (Reading comment) A short tex-code from Overleaf and just edited by me:

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%
% Welcome to Overleaf --- just edit your LaTeX on the left,
% and we'll compile it for you on the right. If you open the
% 'Share' menu, you can invite other users to edit at the same
% time. See www.overleaf.com/learn for more info. Enjoy!
%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\documentclass{book}
\usepackage{blindtext}
\usepackage{hyperref}

\title{Example of Hyperlinks}
\author{Overleaf}

\begin{document}

\frontmatter
\tableofcontents
\clearpage

\addcontentsline{toc}{chapter}{Foreword}
{\huge {\bf Foreword}}

\Blindtext
\clearpage

\addcontentsline{toc}{chapter}{Dummy entry\quad quad}
{\huge {\bf Dummy entry}}

\Blindtext
\mainmatter

\chapter{First Chapter, $a=b$}

This will be an empty chapter

\begin{equation}
\label{eq:1}
\sum_{i=0}^{\infty} a_i x^i
\end{equation}

The equation \ref{eq:1} shows a sum that is divergent. This formula will be used later on page \pageref{second}.

\Blindtext
\clearpage

\section{Second section} \label{second}

\blindtext
\Blinddocument
\end{document}

Answer 1

Unfortunately your code does not create similar errors like you posted, at least in my environment. It may be because you did not use hyperref at all ind the code snippet.

If I go through each warning I've got and "cure" it, I end up with this one, taking some comments made into account:

\documentclass{book}
\usepackage{blindtext}
\usepackage{hyperref}

\title{Example of Hyperlinks}
\author{Overleaf}

\begin{document}

\frontmatter
\tableofcontents
\clearpage

\addcontentsline{toc}{chapter}{Foreword}
{\huge {\bf Foreword}}

\Blindtext
\clearpage

\addcontentsline{toc}{chapter}{Dummy entry}% <<==
{\huge {\bf Dummy entry}}

\Blindtext
\mainmatter

\chapter{First Chapter, equality}% <<==

This will be an empty chapter

\begin{equation}
\label{eq:1}
\sum_{i=0}^{\infty} a_i x^i
\end{equation}

The equation \ref{eq:1} shows a sum that is divergent. This formula will be used later on page \pageref{second}.
\Blindtext
\clearpage

\section{Second section} \label{second}

\blindtext
\Blinddocument
\end{document}

Screenshot:

To localize the source of your warnings I suggest a "homing-in" strategy. I.e. delete one half of your relevant text, rund + check, restore, delete other half, run + check. If you do it right, i.e. matching your situation, one half should pass, while the other will fail. Repeat. // Consider "halfness" as a fuzzy concept. It can be really 50 %, or more, or less. Because you follow a $2^n$ scheme, you should quickly arrive at the trouble-makers.