File -> Import -> DXF / DWG and follow the wizard plus F1 to open help if you don't know the command.
As @cubalibre00 suggested, convert PDF to the DXF/DWG. Import it to the NX part and start the modeling using the imported file.
With Inkspace you can open a PDF file and save it as DXF which can be opened/imported in NX directly.
Sometimes I use Win Topo Freeware to convert content of PDF to image which can be opened in Win Topo and export it as DXF.