I read the documentation for both fancytooltips and fancy-preview here, and I don't really understand what their differences are, or when I should prefer one over the other. I see that fancytooltips is a TeX package (a .sty file), while fancy-preview is a Perl script. But they seem to have the same capability (which is the capability in the pictures on this page.)
Fancytooltips allow to insert pictures (pages of external PDF file) on the page. The user have to prepare the external file and use commands to insert tooltips in the main document.
Fancy-preview creates the external file with tooltips automatically from equations, figures etc and adds the tooltips to
\ref-like commands also automatically.
Both require pdflatex (pdflualatex should be also O.K.), but in principle it is possible to add support also for xelatex.
Similar solutions (add material to PDF page and allow buttons to hide/reveal this material) are
ocgtools and the macro
\tooltip written by AlexG as an answer to pdftooltip from pdfcomment package: Using LaTeX code in tooltip . In these solutions, the TeX code for the tooptip is in the main TeX file.
ocgtools needs pdflatex (is is possible to add support also for xelatex), the other packages and macros allow xelatex or dvips route.
Similar packages are also
pdfcomment. These packages do not allow any LaTeX code in the tooltips, but the support in various browser may be better.