Depending on what program is importing the PDF, a "page" or white bounding box might come along for the ride.
I've run into this when having to use PDF to send artwork from Illustrator or InDesign into Roland VersaWorks. Normally I just save .EPS files from Illustrator. But if the artwork has odd raster effects in it, some floated/transparent layers from Photoshop or other strange things I end up having to save PDF. VersaWorks will bring in the whole page rather than just the art (like it does with EPS). So I just shrink the page size down to fit the artwork closely.