Tooltip that works with all pdf readers
Finally I came up with a \tooltip command that works across a small number of PDF viewers, among which is an Open-Source one. The tooltip command allows for TeX-formatted tip texts.
\tooltip[*[*[*[*]]]] [<link colour>]{<link text>} [<tip box colour>]{<tip text>} [<x-offset>,<y-offset>]
It comes in five variants:
%draggable tip box (e. g. https://tex.stackexchange.com/a/108998),
%visible on mouse-over, hidden on mouse-out
\tooltip{<link text>}{<tip text>}
%draggable tip box, toggle visibility on mouse-over (a second wipe hides the tip)
\tooltip*{<link text>}{<tip text>}
%NON-draggable tip, visible on mouse-over, hidden on mouse-out
\tooltip**{<link text>}{<tip text>}
%NON-draggable tip, toggle visiblity on mouse-over
\tooltip***{<link text>}{<tip text>}
%NON-draggable tip, toggle visiblity on mouse-click
\tooltip****{<link text>}{<tip text>}
For Evince (open-source), only the command with 4 stars \tooltip**** is usable, because mouse-click is the only event Evince listens for. In Acrobat Reader, all variants are functional.
The non-draggable variants (two and more stars) do not use any JavaScript.
If hyperref is loaded, the colour of internal links (hyperref option linkcolor) is used as <link colour>.
In order to break longer <tip text> into multiple lines, wrap it in a \parbox of given width:
\tootltip{link text}{\parbox{0.5\linewidth}{... long tip text ...}}
Evince:
A-Reader:
\documentclass[a6paper,12pt]{scrbook}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%
% tooltips with LaTeX v. 2019/09/26
%
% \tooltip[*[*[*[*]]]]
% [<link colour>]{<link text>}
% [<tip box colour>]{<tip text>}
% [<x-offset>,<y-offset>]
%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%
% \tooltip --> draggable tip, visible on mouse-over, hidden on mouse-out
%
% \tooltip* --> draggable tip, toggle visiblity on mouse-over
%
% \tooltip** --> NON-draggable tip, visible on mouse-over, hidden on mouse-out
%
% \tooltip*** --> NON-draggable tip, toggle visiblity on mouse-over
%
% \tooltip**** --> NON-draggable tip, toggle visiblity on mouse-click (Evince!)
%
% Default link colour can be set with
%
% \usepackage[linkcolor=<colour>]{hyperref}
%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\usepackage{pdfbase}[2017/03/16]
\usepackage{xparse,ocgbase}
\usepackage{xcolor,calc}
\usepackage{tikzpagenodes,linegoal}
\usetikzlibrary{calc}
\usepackage{tcolorbox}
\ExplSyntaxOn
\let\tpPdfLink\pbs_pdflink:nn
\let\tpPdfAnnot\pbs_pdfannot:nnnn\let\tpPdfLastAnn\pbs_pdflastann:
\let\tpAppendToFields\pbs_appendtofields:n
\def\tpPdfXform{\pbs_pdfxform:nnnnn{1}{1}{}{}}
\let\tpPdfLastXform\pbs_pdflastxform:
\let\cListSet\clist_set:Nn\let\cListItem\clist_item:Nn
\ExplSyntaxOff
\makeatletter
\NewDocumentCommand{\tooltip}{%
ssssO{\ifdefined\@linkcolor\@linkcolor\else blue\fi}mO{yellow!20}mO{0pt,0pt}%
}{{%
\leavevmode%
\IfBooleanT{#2}{%
%for variants with two and more stars, put tip box on a PDF Layer (OCG)
\ocgbase@new@ocg{tipOCG.\thetcnt}{%
/Print<</PrintState/OFF>>/Export<</ExportState/OFF>>%
}{false}%
\xdef\tpTipOcg{\ocgbase@last@ocg}%
%prevent simultaneous visibility of multiple non-draggable tooltips
\ocgbase@add@ocg@to@radiobtn@grp{tool@tips}{\ocgbase@last@ocg}%
}%
\tpPdfLink{%
\IfBooleanTF{#4}{%
/Subtype/Link/Border[0 0 0]/A <</S/SetOCGState/State [/Toggle \tpTipOcg]>>
}{%
/Subtype/Screen%
/AA<<%
\IfBooleanTF{#3}{%
/E<</S/SetOCGState/State [/Toggle \tpTipOcg]>>%
}{%
\IfBooleanTF{#2}{%
/E<</S/SetOCGState/State [/ON \tpTipOcg]>>%
/X<</S/SetOCGState/State [/OFF \tpTipOcg]>>%
}{
\IfBooleanTF{#1}{%
/E<</S/JavaScript/JS(%
var fd=this.getField('tip.\thetcnt');%
if(typeof(click\thetcnt)=='undefined'){%
var click\thetcnt=false;%
var fdor\thetcnt=fd.rect;var dragging\thetcnt=false;%
}%
if(fd.display==display.hidden){%
fd.delay=true;fd.display=display.visible;fd.delay=false;%
}else{%
if(!click\thetcnt&&!dragging\thetcnt){fd.display=display.hidden;}%
if(!dragging\thetcnt){click\thetcnt=false;}%
}%
this.dirty=false;%
)>>%
}{%
/E<</S/JavaScript/JS(%
var fd=this.getField('tip.\thetcnt');%
if(typeof(click\thetcnt)=='undefined'){%
var click\thetcnt=false;%
var fdor\thetcnt=fd.rect;var dragging\thetcnt=false;%
}%
if(fd.display==display.hidden){%
fd.delay=true;fd.display=display.visible;fd.delay=false;%
}%
this.dirty=false;%
)>>%
/X<</S/JavaScript/JS(%
if(!click\thetcnt&&!dragging\thetcnt){fd.display=display.hidden;}%
if(!dragging\thetcnt){click\thetcnt=false;}%
this.dirty=false;%
)>>%
}%
/U<</S/JavaScript/JS(click\thetcnt=true;this.dirty=false;)>>%
/PC<</S/JavaScript/JS (%
var fd=this.getField('tip.\thetcnt');%
try{fd.rect=fdor\thetcnt;}catch(e){}%
fd.display=display.hidden;this.dirty=false;%
)>>%
/PO<</S/JavaScript/JS(this.dirty=false;)>>%
}%
}%
>>%
}%
}{{\color{#5}#6}}%
\sbox\tiptext{%
\IfBooleanT{#2}{%
\ocgbase@oc@bdc{\tpTipOcg}\ocgbase@open@stack@push{\tpTipOcg}}%
%\fcolorbox{black}{#7}{#8}%
\tcbox[colframe=black,colback=#7,size=fbox,arc=1ex,sharp corners=southwest]{#8}%
\IfBooleanT{#2}{\ocgbase@oc@emc\ocgbase@open@stack@pop\tpNull}%
}%
\cListSet\tpOffsets{#9}%
\edef\twd{\the\wd\tiptext}%
\edef\tht{\the\ht\tiptext}%
\edef\tdp{\the\dp\tiptext}%
\tipshift=0pt%
\IfBooleanTF{#2}{%
%OCG-based (that is, all non-draggable) boxes should not extend beyond the
%current column as they may get overlaid by text in the neighbouring column
\setlength\whatsleft{\linegoal}%
}{%
\measureremainder{\whatsleft}%
}%
\ifdim\whatsleft<\dimexpr\twd+\cListItem\tpOffsets{1}\relax%
\setlength\tipshift{\whatsleft-\twd-\cListItem\tpOffsets{1}}\fi%
\IfBooleanF{#2}{\tpPdfXform{\tiptext}}%
\raisebox{\heightof{#6}+\tdp+\cListItem\tpOffsets{2}}[0pt][0pt]{%
\makebox[0pt][l]{\hspace{\dimexpr\tipshift+\cListItem\tpOffsets{1}\relax}%
\IfBooleanTF{#2}{\usebox{\tiptext}}{%
\tpPdfAnnot{\twd}{\tht}{\tdp}{%
/Subtype/Widget/FT/Btn/T (tip.\thetcnt)%
/AP<</N \tpPdfLastXform>>%
/MK<</TP 1/I \tpPdfLastXform/IF<</S/A/FB true/A [0.0 0.0]>>>>%
/Ff 65536/F 3%
/AA <<%
/U <<%
/S/JavaScript/JS(%
var fd=event.target;%
var mX=this.mouseX;var mY=this.mouseY;%
var drag=function(){%
var nX=this.mouseX;var nY=this.mouseY;%
var dX=nX-mX;var dY=nY-mY;%
var fdr=fd.rect;%
fdr[0]+=dX;fdr[1]+=dY;fdr[2]+=dX;fdr[3]+=dY;%
fd.rect=fdr;mX=nX;mY=nY;%
};%
if(!dragging\thetcnt){%
dragging\thetcnt=true;Int=app.setInterval("drag()",1);%
}%
else{app.clearInterval(Int);dragging\thetcnt=false;}%
this.dirty=false;%
)%
>>%
>>%
}%
\tpAppendToFields{\tpPdfLastAnn}%
}%
}}%
\stepcounter{tcnt}%
}}
\makeatother
\newsavebox\tiptext\newcounter{tcnt}
\newlength{\whatsleft}\newlength{\tipshift}
\newcommand{\measureremainder}[1]{%
\begin{tikzpicture}[overlay,remember picture]
\path let \p0 = (0,0), \p1 = (current page.east) in
[/utils/exec={\pgfmathsetlength#1{\x1-\x0}\global#1=#1}];
\end{tikzpicture}%
}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{document}
Einstein's \tooltip****{formula}{$E=m c^2$} is well known.
Another famous formula is due to \tooltip****{Pythagoras}{$a^2+b^2=c^2$}.
This \tooltip{tip}{is visible only in AR} is draggable and shown on mouse-over.
\end{document}
Tooltips in PDF documents are generally possible. I do know two packages; one of them you tagged yourself for your question, the other one I added, click on these tags to see, which questions were already asked in TeX.SX: fancytooltips & pdfcomment. But as Joseph Wright in a comment wrote:
How PDF viewers show 'hover text' is down the them, not the source (LaTeX or otherwise). Thus the best you can hope is to check with a set of viewers.
I can tell you, that SumatraPDF, a popular viewer for Windows, does not show any tooltip at all with “tooltip” in a strict meaning, but does show comments, when you hover with the mouse over one of them. What is supported, what not, can be seen when reading the documentation of package pdfcomment. There is a type \pdfmarkupcomment that actually almost acts like a tooltip.
See also this fixed bug in SumatraPDF Bug tracker: PDF Comments Don't Work.