Hello Randy11,
Thanks for your reply.
I found out that with a long-press of a finger, I'm able to invoke the highlighting dialog. (Although it is annoying to wait for so long.)
However, using the stylus, I can only draw - even when long pressing somewhere for a while: take the tip of the stylus, place it on top of a written word, wait without moving.
I checked on my normal computer: the PDF document does feature a word layer and does not only consist of an image layer.
It is awkward to ask but is there something I could do wrong with long-pressing via stylus?