Text copied from pdf is missing spaces, or has extra ones

This is at least partly a known issue with Adobe Reader. Adobe Reader fails to recognise spaces between words in certain cases (e.g. where the spacing is smaller than average) or recognises one space as multiple spaces (e.g. where the spacing is larger than average).

It is an issue in the viewer, not the file - as demonstrated by the fact that other viewers work fine - and there's not much to be done on the TeX side of things.

Old thread, but I had the same problem as you and after a lot of searching online I was able to find a fix that really helped me.

\fontdimen2 controls the spacing between words in LaTeX so we can use that to make sure a PDF reader can tell where one word starts, and another ends.

Here's a simple example using an environment to define an area that we want different spacing in:

\documentclass{article}
\usepackage{newtxtext}

\newenvironment{goodspacing}{
    \fontdimen2\font=0.5em
}

\begin{document}
\begin{goodspacing}
    Therefore, this work ...
\end{goodspacing}
\end{document}

Wrapping your content with this environment should be sufficient enough for copy and pasting to work correctly.

Adjust the number after \font= to your desired amount of inter-word spacing.