Rights Management will block the use of the Windows native screen capture capability (Ctrl + PrtSc), but it cannot prevent screen captures from trird party utilities such as Snag it, or the new Windows Snipping Tool.
Unless the computer used to display the PDF is in a secure room where users are prevented from bringing in cell phones or digital cameras, there are ways to get a “picture” of the screen.
Rights Management has a “Dynamic Watermark” feature feature (i.e. displays the user name or ID) which can help discourage the possibility of the user wanting to distribute a screen capture of the protected document.
Regards
Steve