As George says, it is easy enough to display a popup alert message. But you can't dynamically populate a PDF with content on the client side. At least you can't do it in a reliable and efficient maner.
You can see an example of this type of solution here:
Scroll down to the "Splash Screens" - Document License Example.
Here's an article on controling OCGs with a script:
The source for PDF Scripting Info
Having trouble, Why Doesn't my Script Work?