We open PDF docs located in the DAM - we simply include target="_blank":
<a target="_blank" href="https://forums.adobe.com/content/dam/help/attachments/dev5.pdf">Developing for Adobe Experience Manager</a></p>
Also - most CQ components also give you an option to open the content in a new window:
[img]newWin.png[/img]
I have never seen an issue opening a PDF in a new window.