Can I edit PDF files with Scribus?

No.

PDF as file format was never ever meant to editable in the conventional sense. Internally it is non-linear in layout and is optimised for compactness. It is better to think of PDF as electronic paper which is printed. You cannot 'unprint' paper easily can you ?

There are o­nly two applications which can "edit" PDF. Enfocus Pit Stop Pro and Quite.com's Box of Tricks are special pre-press plugins for Acrobat Pro. Using these is best if you have a fair amount of knowledge about pre-press and PDF.

See: http://rants.scribus.net/2006/12/10/pdf-surgery/