documentclass{article}
usepackage{lipsum}
usepackage{tikz}
usetikzlibrary{tikzmark}
begin{document}
lipsum[1]

tikzmark{a}Hello world!tikzmark{b}

end{document}

relax
savepointas{a}{pgfid1}{0pt}{0pt}
savepicturepage{pgfid1}{1}
pgfsyspdfmark {pgfid1}{9782558}{33796915}
savepointas{b}{pgfid2}{0pt}{0pt}
savepicturepage{pgfid2}{1}
pgfsyspdfmark {pgfid2}{13243229}{33796915}

pgfsyspdfmark{pgfid<n>}{<x>}{<y>}

documentclass{article}
usepackage{lipsum}
usepackage{tikz}
usetikzlibrary{tikzmark,calc}
begin{document}
lipsum[1]

tikzmark{a}Hello world!tikzmark{b}

tikz[overlay,remember picture]{
draw[latex-] let p1=($(pic cs:a)-(current page.south west)$),
n1={978.2558/x1},n2={3379.6915/y1} in
(pic cs:a) -- ++(0,-1) node[below] {$(x1,y1)to(n1,n2) $};
draw[latex-] let p1=($(pic cs:b)-(current page.south west)$),
n1={1324.3229/x1},n2={3379.6915/y1} in
(pic cs:b) -- ++(6,-1) node[below] {$(x1,y1)to(n1,n2) $};
}
end{document}