Does the Preview app have a freehand pen/pencil tool?
Yes, Preview in OS X 10.10 (Yosemite) has a freehand tool.  Access it within Preview from the "Markup Toolbar" icon of the .JPG or .PDF file you have open.


  


When you click on that icon, the Markup toolbar appears, and the freehand "sketch" tool is the 3rd icon from the left, in blue in the image below.


  


and it offers a smoothing option as well as a freehand mode.