When you use Save As > Save Copy As > PDF, or Export > PDF, Inventor always opens the PDF viewer to show the PDF file after it is published. This can be a bother, due to the delay caused by waiting for the PDF viewer to open each time.
The current solution is to go to Export > Export to DWF and un-check the Display Published File in Viewer option found at the bottom of the General tab. This option controls the PDF viewer as well as the DWF. With this option unchecked the PDF will not open automatically after it is published.
However, this setting is not at all discoverable and also precludes the ability to set the DWF and PDF viewer open options independently. Adding an option in the PDF options dialog would resolve this. There is currently a request to add customizable default settings to the PDF export here: