I would like to know how many pages does your PDF consist of?
Currently there is a little problem to import a PDF whose page number is larger than 500. We have logged this problem and fixed for future release.
I hope this can be of help.
The PDF was only 20 pages or so not nearly so big as 500. They were camera views from Revit. When we had tried including the camera views in the dwf export, they changed the angle or the lens when brought into Design Review. Bringing in the pdf was a work around for the other Design Review issue.