Use trackpad for annotation in Preview without clicking
If your MacBook has a Force Touch trackpad, you can use the Draw tool instead of the Sketch tool.


  Draw 
  
  Draw a shape using a single stroke. Press your finger more firmly on the trackpad to draw with a heavier, darker line.
  
  This tool appears only on computers with a Force Touch trackpad.


https://support.apple.com/kb/PH20218