How to magnify pre-selected text?

Question

I want to be able to magnify (by say 2X) a group of equations in the output of PDFLaTeX by a single click on the box containing them. A second click is to revert to the previous state. The viewer is Adobe Reader.

Answer 1

One solution would be to add two copies of the page to the document, one with the small formula and one with the enlarged formula. Then you can use hyperlinks to switch between the pages, effectively toggling the formula between the small and big version. As a proof of concept, try the TeX code below.

Here is the initial document:

Clicking on the formula yields:

Clicking again returns to the first image.

\documentclass{article}

\usepackage{hyperref}
\usepackage{amsmath}

\begin{document}
Click on the formula to make it bigger:
\hyperlink{page.2}{\begin{align*}
  d\bigl(f(t, X_t)\bigr)
  &= \frac{\partial}{\partial t} f(t, X_t) \,dt
    + \sum_{i=1}^d \frac{\partial}{\partial x_i} f(t, X_t) \,dX^{(i)}_t \\
  &\hskip2cm
    + \frac12 \sum_{i,j=1}^d \frac{\partial^2}{\partial x_i \partial x_j} f(t, X_t)
      \,dX^{(i)}_t \, dX^{(j)}_t,
\end{align*}}
Some more text could be here.

\newpage

Click on the formula to make it smaller:
\hyperlink{page.1}{\Large\begin{align*}
  d\bigl(f(t, X_t)\bigr)
  &= \frac{\partial}{\partial t} f(t, X_t) \,dt
    + \sum_{i=1}^d \frac{\partial}{\partial x_i} f(t, X_t) \,dX^{(i)}_t \\
  &\hskip2cm
    + \frac12 \sum_{i,j=1}^d \frac{\partial^2}{\partial x_i \partial x_j} f(t, X_t)
      \,dX^{(i)}_t \, dX^{(j)}_t,
\end{align*}}
Some more text could be here.    
\end{document}

The extra page(s) could be hidden at the end of the document to not disturb the normal sequence of pages.