This has been a problem for me since 10.4.0.
I go to Tools > Get Info and try to enter keywords under the "keywords" tab. But my keywords won't even show up after I enter them. They literally disappear if I switch to another tab or close the dialog box.
Now the keyword tool in preview works fine for tagging images. But PDF's (whether I created them or from somewhere else) always seem to give me trouble.
Does anyone else experience this weirdness?
P.S. The PDF's I am trying to add keywords to ARE NOT password protected.