This feature used to work fine. I just tried to use the sketch/freehand tool in Preview to annotate a PDF and while the little "pen point" shows up in blue, the area I sketch remains blank, with only the parameters of the sketch showing (those little dots that show the size of the annotation).
I've made sure that my background color is set to nothing.The little vertical tab that turns up under the Aa on the far left shows the doodle, but clicking on the various options does not reveal the doodle on my PDF. Right clicking on the doodle area offers the option "Send to back," not "Send to front."
I am running Sierra 10.12.6 on an iMac.
How can I get this to work properly?