Is it possible to have multiple icons open for multiple instances of an application? For example, if I have two pdf's open, can I have two "preview" icons open in the doc bar?
You can open multiple instances of a mac application using
open -n terminal command. Eg.
open -n image2.pdf
Because I have to do that quite often : Yes, it is indeed possible and it makes use of the command provided in the answer by kekub.
Just create an application with Automator:
1) Open Automator
2) Choose "Application"
3) Search for the Shell-Script workflow element
4) Change the Input to "As Arguments"
5) Adjust the Shell script like this :
for f in "$@" do open -a preview -n "$f" done
6) Save the Programm as Something like "Preview+"
7) Optional : Change the default application for images to this created app
8) Optional : Move it to the dock and drop images on it.
Now for each Image there will be a new instance of Preview.
You can do that with every app you like.