Insert a PDF using Insert, Object, Create from File and pick the PDF.
Rightclick the inserted PDF, choose Action Settings.
In the MouseClick tab of the dialog box, choose Object Action.
Open will probably be the only available choice in the dropdown. That's what
you want.
Click OK.
The PDF should now open when clicked in Slide Show view, leaving the PPT show
open behind it.
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PPT Frequently Asked Questions
http://www.pptfaq.com/
PPTools add-ins for PowerPoint
http://www.pptools.com/
You're very welcome. Now pay it forward. <g> Pass the info along to the people
you've seen fighting with it. Thanks!