Hey, as I was curious if I could replace the bundled pdf.js viewer with my own, and I needed to familiarize myself with pdf.js and obsidian plugin dev anyway for other things, I managed to get a proof of concept up and running using the standard latest npm version of pdf.js inside obsidian.
The blurriness can be completely removed by changing a parameter given to the pdf object from PDFJS.getDocument. When calling getViewport in pdf.js, this can be adjusted.
I.e. const viewport = pdfPage.getViewport({ scale: 4.0 }); and rendering the pdf via an attached div on the current page.
Everything is completely clear to me now. Given I know obsidian has some more stuff on top of it, so it might not be so simple, but I will further explore this option later and see if I can bundle it as a quickfix via a plugin at some point, unless of course they fix it first 
Otherwise it might be that this scaling factor is not being set high enough, when the layout logic and similar is getting called natively in obsidian. But it might be other issues, that I am not simply aware of, since I am new to obsidian plugin dev etc.