This addin allows simple import of PDFs directly into the Navisworks 2015 Project Browser as sheets that can be reviewed, marked up and quantified. Previously, to accomplish this you could open the PDF in Design Review with vector information, save as DWF and "Import Sheets and Models" to bring the DWF sheet into Navis. As you may know, the 2015 release introduced a lot more Quantification functionality for 2D, and I believe this new PDF importing tool will become a key part of that workflow.
In short, it seems that the sheet import process for Navisworks 2D documets is about to get a lot easier!
Link to PDF Reader download page (sign in with your Subscription account login on Exchange and then the Subscription only button will change to "Free" and you can download AutodeskNavisworks2015PDFReader.msi)
Read the help document here
The entire Navisworks App Exchange is here
I hinted at the document aggregation possibilities in Navisworks back in this post.