I've just converted an old paper with ps2pdf but while the result is legible, searching still does not work. It seems there are some invisible characters splitting the words.
Any advice on how to do the conversion differently?
That PS file still uses the original (Metafont) bitmap fonts, and after conversion to PDF with ps2pdf you'll see something like this:
Such PDF files are not searchable (see e.g. Are Knuth's fonts searchable in pdf yet?).
Since the source isn't available you can use the pkfix perl script (CTAN link) by Heiko Oberdiek (distributed with TeXLive; I don't know about MikTeX): it tries to replace the Type 3 fonts by the corresponding Type 1. Luckily it works on your file, so the combination
> pkfix 2003-069.ps 2003-069-fixed.ps
> ps2pdf 2003-069-fixed.ps
results in a file 2003-069-fixed.pdf with Type 1 fonts
which is searchable.
\usepackage[T1]{fontenc}. But that's a guess, we need to see some code. – campa Jan 26 '21 at 11:47