#1 In latest versions like 8.1 and versions above that: can you attach a pdf to a case attachment in OOTB attachments section.
#2 once PDF is attached, then click on the link from attachments section to preview it.
->Good part with preview is: you will see a modal dialog covering your screen, but you can close the modal dialog/overlay and continue with case and action it. In a way this is powerful. And doesn't need any customisation.
You can look at this code on how this is achieved. If you still feel that you want it to open in new window altogether:
- As Rincy mentioned: Try "Open URL in new window",and have the logic in your activity. And this logic should similar to what Pega is acieving in OOTB scenario in versions > 8.1. I have tried this and it works for me.