Using preview, it’s very easy to digitally ‘sign’ a document. This will save you the step of having to print it out, sign it, and scan it. Of course, use your discretion when using this trick (as in, I’m fairly certain passports and birth certificates might not be appropriate!). What’s extra cool is that in Yosemite you can use your trackpad to sign!
1) Open preview.
2) Find and click the Toolbox Icon: 
3) Look for an icon that looks like a signature: 
4) You can now choose to either use your trackpad to write your signature, or to follow the directions to ‘copy’ a version that you physically write on a piece of paper.