Any way to change existing font in a pdf?

It is not so easy as it might be… The pdf has probably been optimized to place each group of glyphs at a specific position, so that changing the font would break most of the composition.

See this:

https://superuser.com/questions/166024/substitute-a-font-in-...

It does not solve your problem, though.

PDF is intended to lock-in page layouts.

If it really mattered, my first step would probably be treating the PDF as an image and convert the image to text.

There are certain tools available online like Adobe Acrobat or PDF2Go.

unfortunately they only allow adding new text, among other features, none of which is very helpful for my need