Hi Carnea,
Is there a specific reason for cropping the pdf instead of trying to get the elements i.e. type, photo, illustration out of the pdf file and use directly within Scribus...I know that this would be my first preference when working with artwork. Also which version of Scribus and Operating System are you using and I can give you suggestions for apps to accomplish my suggestion.
BTW, you need to import the pdf file into an image frame and you should be able to resize the frame to hide the elements that you do not want to see. I just did it yesterday for a quick mock layout for a customer to see and then I recreated it fully today by pulling out the elements I needed directly out of the pdf file and into Scribus.
mnawij