For coders only 🙂
Note1: most of these examples are copyrighted Adobe® and are available online here with much more details.
Note2: PDF actions are working with Acrobat Reader® from Adobe®. Support for PDF Actions in any alternative readers may produce unknown results.
Open MessageBox with custom text
Playing a sound clip from a URL
Although a bit more complex than playing a local file, this insures that file exists.
Adding page transitions
Obviously to be triggered at PDF opening