The reason you may not be viewing your PDF proof properly is a result of a setting in your Adobe Acrobat (or Adobe Reader) needs to be adjusted.
I believe when creating your artwork, the artist made a multi-channel Adobe Photoshop document that has CMYK plus Spot channels. In order to preview this properly and see the content you need to turn on your Overprint Preview in Acrobat Reader.
Overprint Preview is turned off by default (both in Acrobat and the free Adobe Reader) and must be activated in the preferences of each application.
Open up Acrobat (or Reader) then go to Preferences>General (Mac) or Edit menu then Preferences (PC).
Next, select Page Display from the list.
In the Use Overprint Preview pull-down menu, choose Always.
