I don't think that you can change icons inside a pdf, but you can create an icon, place it inside a folder you control and change the properties of the launch shortcut to point to said icon.
For manipulating icons I use an old tool : icoFX. I'm sure you can find other tools.
Once you have your icon, use the shortcut properties to reassign it to your shortcut.
I've got nothing to hide, and I demand that you justify what right you have to ask.