On Ubuntu 20.04, I've texlive 2021 installed and compile the following tex example file with latexmk:

%%%%%%%%
\documentclass{paper}

\usepackage{amsmath}
\usepackage{mathtools}
\usepackage{braket}
\usepackage{stix}
\begin{document}

$\hat{\mathcal{H}}=-\frac{\hbar^{2}}{2 m} \int_{-\infty}^{\infty} \mathrm{d} x|x\rangle \frac{\mathrm{d}^{2}}{\mathrm{~d} x^{2}}\left\langle x\left|\otimes \mathbb{1}+\int_{-\infty}^{\infty} \mathrm{d} x\right| x\right\rangle V_{0}(x)\left\langle x\left|\otimes \mathbb{1}+\int_{-\infty}^{\infty} \mathrm{d} x\right| x\right\rangle V_{z}(x)\langle x| \otimes \hat{S}_{z}$
$\rangledownzigzagarrow$

\end{document}

%%% Local Variables:
%%% mode: latex
%%% TeX-master: t
%%% End:
%%%%%%%%

The `~/.latexmkrc' file on my side is shown below:

###
#!/usr/bin/env perl
$pdf_mode = 3;

$latex = "uplatex -synctex=1 -halt-on-error -interaction=nonstopmode %O %S";
$dvipdf = 'dvipdfmx %O -o %D %S';
$bibtex = 'upbibtex %O %S';
$makeindex = 'mendex %O -o %D %S';
$max_repeat = 5;
###

The generated PDF file is located at <https://github.com/hongyi-zhao/temp/blob/master/latex/test.pdf>. I find that some mathematical symbols in the generated PDF cannot be copied correctly. For example, the character in the PDF file corresponding to the `\hat{\mathcal{H}}' command in the source LaTeX file, when copying and pasting, will eventually become like this:  ̂ .

Any hints for this problem will be highly appreciated.

Regards,
HY

On 7/7/21 10:43 PM, hongy...@gmail.com wrote:
> I find that some mathematical symbols in the generated PDF cannot be copied correctly. For example, the character in the PDF file corresponding to the `\hat{\mathcal{H}}' command in the source LaTeX file, when copying and pasting, will eventually become like this:  ̂ .
>
> Any hints for this problem will be highly appreciated.

Personally, I'm more surprised when copy-and-paste of mathematical text
works than when it doesn't.

You can PDF's ActualText feature to specify the precise text that should
go into the clipboard. I don't remember the exact syntax offhand or if
there's a package that simplifies this (tagpdf, maybe?), but it looks
something like this:

\pdfliteral{
/Span << /ActualText (2 + 2 = 4 blah blah your text here) >>
BDC
}%
$\hat{\mathcal{H}}=-\frac{\hbar^{2}}{2 m} \int_{-\infty}^{\infty}
\mathrm{d} x|x\rangle \frac{\mathrm{d}^{2}}{\mathrm{~d}
x^{2}}\left\langle x\left|\otimes \mathbb{1}+\int_{-\infty}^{\infty}
\mathrm{d} x\right| x\right\rangle V_{0}(x)\left\langle x\left|\otimes
\mathbb{1}+\int_{-\infty}^{\infty} \mathrm{d} x\right| x\right\rangle
V_{z}(x)\langle x| \otimes \hat{S}_{z}$
\pdfliteral{EMC}

-- Scott

On Friday, July 9, 2021 at 3:39:34 AM UTC+8, Scott Pakin wrote:
> On 7/7/21 10:43 PM, hongy...@gmail.com wrote:
> > I find that some mathematical symbols in the generated PDF cannot be copied correctly. For example, the character in the PDF file corresponding to the `\hat{\mathcal{H}}' command in the source LaTeX file, when copying and pasting, will eventually become like this:  ̂ .
> >
> > Any hints for this problem will be highly appreciated.
> Personally, I'm more surprised when copy-and-paste of mathematical text
> works than when it doesn't.
>
> You can PDF's ActualText feature to specify the precise text that should
> go into the clipboard. I don't remember the exact syntax offhand or if
> there's a package that simplifies this (tagpdf, maybe?), but it looks
> something like this:
>
> \pdfliteral{
> /Span << /ActualText (2 + 2 = 4 blah blah your text here) >>
> BDC
> }%
> $\hat{\mathcal{H}}=-\frac{\hbar^{2}}{2 m} \int_{-\infty}^{\infty}
> \mathrm{d} x|x\rangle \frac{\mathrm{d}^{2}}{\mathrm{~d}
> x^{2}}\left\langle x\left|\otimes \mathbb{1}+\int_{-\infty}^{\infty}
> \mathrm{d} x\right| x\right\rangle V_{0}(x)\left\langle x\left|\otimes
> \mathbb{1}+\int_{-\infty}^{\infty} \mathrm{d} x\right| x\right\rangle
> V_{z}(x)\langle x| \otimes \hat{S}_{z}$
> \pdfliteral{EMC}
>

Tried the above code, but failed:

ERROR: Undefined control sequence.

--- TeX said ---
l.12 \pdfliteral
{
--- HELP ---
TeX encountered an unknown command name. You probably misspelled the
name. If this message occurs when a LaTeX command is being processed,
the command is probably in the wrong place---for example, the error
can be produced by an \item command that's not inside a list-making
environment. The error can also be caused by a missing \documentclass
command.

> -- Scott

On 7/8/21 6:51 PM, hongy...@gmail.com wrote:
> Tried the above code, but failed:
>
> ERROR: Undefined control sequence.
>
> --- TeX said ---
> l.12 \pdfliteral
> {

\pdfliteral is defined by pdfLaTeX. If you're using LuaLaTeX, replace
"\pdfliteral{...}" with "\pdfextension literal{...}". If you're using
XeLaTeX, then I'm not sure, but I think it's something like
"\special{pdf:literal ...}". Perhaps someone else can confirm that.

-- Scott