The cool thing about PostScript (the underlying imaging model for pdf) is that it is "resolution independent" because it stores text and diagrams as Geometry. You can scale it up as much as you like, and it will always be totally crisp.
However, PostScript (and therefore pdf) can also embed regular pixel images that have a fixed resolution.
If you find a pdf that is "fuzzy", in all likelihood, you've simply got a pdf containing an image.
And what that means, is you cannot convert it to a (geometric format) like dxf. π’
Adam