Force Preview to use freehand mode by default
By default the drawing engine in Preview can't be changed (in El Capitan or earlier) as it's a vector base drawing system. So it will always have some level of smoothing applied to it based on the algorithm Apple is using.

You can however turn off some of the smoothing for documents that store the drawing in vector format like PDF, but this will have a minimal effect at best for most drawings.

If you want to try turning off the vector smoothing under Mac OS X 10.11 El Capitan.

Go into the Preview -&gt; Preferences... and choose the third tab PDF.

Un-checkmark Smooth text and line art.



Beyond that there isn't a way to turn off the math that drives the drawing tool in Preview.