The main problem is the rasterization. There's an amazing tool to fix this, called pkfix:

    wget "https://cs.uwaterloo.ca/~shallit/Papers/change2.ps"
    pkfix change2.ps change2-fixed.ps
    ps2pdf change2-fixed.ps

Result: https://shreevatsa.net/post/2023-pkfix/change2-fixed.pdf

Side-by-side: https://shreevatsa.net/post/pkfix/ (Barebones post with screenshot as I need to go now; will add more details later today.)

Blame Knuth. Afaik the font used simulates closely the typesetting of the original art of programming edition that was printed using hot metal typesetting on an old monotype machine. Later these machines were eliminated for more modern techniques but he didn’t like the look so developed TeX to achieve a fidelity with those machines for art of computer programming later editions and new volumes. It has been almost universally adopted in academia since.

It's a paper intended for printing, maybe not that suitable for scaling. I like this font.

It's largely used as a status symbol to show that you know how to code in LaTeX.

That said, something is wrong with this PDF, it looks like it has been rasterized somewhere in the process, which is not normal. LaTeX usually outputs nice clean curves on the text.

Every Latex editor I've used uses (I think) that font by default

Serifs were still kinda cool in 2002, I guess