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
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
Similar packages are also