This question was asked here, but the suggested software,
[smoothscan], seems to be abandoned (latest commit is dated 2013, 5 years ago).
The other solution, any2djvu, needs to run through a web server.
Is there a linux equivalent to Adobe ClearScan?
ClearScan is a technology that makes the text of the pdf look smoother, not by substituting it with a system font but by creating a new font which is incorporated into the PDF. Here for more details.
And I would like to have a software that makes the glyphs smoother in order for them to be more readable. Vectorizing the glyphs would also be nice so that they would not become edgy once zoomed in, but is not strictly required. This is what Adobe ClearScan does. I run Ubuntu 18.10 with the KDE Environment.
I would like a free/open source software but I am also available to pay a small fee (20$).