There’s some apps you just don’t bother replacing.
Preview for one.
Acrobat has tons more features but it’s slow and over-complex and takes an age to load. It’s not really for just looking at stuff, it’s about doing some work. I always feel that the real app is the Pro version and the cut down version has to carry the baggage of an app with a lot to do. I only really use it for particular documents which have tons of layers in them, the IKEA catalogue comes to mind. Preview sometimes drops the ball on documents which are difficult to render and leaves a layer or two out...
But other than that, Preview is nimble and accurate and does the job for 99% of PDFs and also images of all formats. Why change it? I never thought I would until Skim came out the other day.
I used Skim for two minutes, then quit, selected a PDF in Path Finder, chose Get Info and set the default app for all PDFs to Skim.
It can match Preview in terms of speed and it’s attractive and uncluttered. When you use it, it becomes clear that whoever designed it had a focus on what people need when they want to look at stuff and you know, read...
So, in no particular order...
it’s got a quick magnify tool, a cool floating window which can zoom in on different parts of a page.
You can use the snapshot feature to open up multiple windows on a document.
It’s got lots of annotation options, more than Preview, and you can add anchored notes while you’re at it. It has bookmarking so you can set placeholders for the key points that interested you.
It’s got nifty Fullscreen and Presentation modes which I can see being used as a basic Powerpoint or Keynote replacement. So once they fix the slightly goofy document icon, it’s near perfect.
And being open source... it’s Free.