Welcome to SolidWorks forums Anne.
Not sure about the reason so can't suggest you anything on that. But can definitely suggest to try edrawings (if you've not yet). The team who is going to view these edrawings can download the free edrawings viewer and have better visualization than 3d PDF. Plus you can get more option with edrawings.
You might also find this post helpful for your future reference.
I am pretty sure the 3D pdf does not carry the RealView material and lighting information. It just contains the colors of the bodies with no lighting making the 3D pdf appear very dark.
You might try eDrawings instead or color your parts in your SolidWorks file differently for your 3D pdf. Create a Display State to use for 3D pdf's.
I also beleive there are options in Adobe to add lighting to the 3D pdf. Not sure as it has been awhile since I played with 3D pdf's. At least there were when Adobe first came out with the Adobe Acrobat 3D back in the day.
Thank you both for your response...can you tell me more about this eDrawing??
The purpose of the 3d pdf is for outside salesman (usually with an ipad or laptop) can access this pdf and rotate the model showing it to the potential customer.
We also want to include information about the model below it.
Would they be able to do that with eDrawing??
eDrawings are the thing then. There is an app for eDrawings on the App Store that will do what you want for your sales guys.
In SolidWorks you can do a File > Save As to eDrawings. eDrawings is part of your SolidWorks install.
You can download for a laptop from the SolidWork Customer Portal Download section. It is under the Free Products tab in the Download section.
You can have markups in the edrawings (you can add them for customers plus your sales can add customer feedback) plus if there are animation of your models, those can be saved with the edrawings and played within the edrawings.