Myself, I have only little web development experience and don't know how hard it would be to port an app like skim, but I could try if given some instructions. Meanwhile I'm interested to know how much would it cost to sponsor it myself, so other people can build it quicker. I'm interested to port it to Arch distro.
This is absolutely not feasible. It is too much based on OSX frameworks. It would be just a completely different program that may (or may not) work a bit like Skim. So it would be made by a Linux developer who is inspired by Skim. That's not not an option for us. Apart from that, the data depends on OSX frameworks, so it would be very hard if not impossible to port that to Linux (it doesn't even port to iOS).