Once the layout is done and you've checked with a print output if it is OK, you can send your document to a professional print provider. They used to ask for an EPS file in the past but PDF is now really becoming the default. You shouldn't have to send the Scribus document unless there is a real problem with your layout. Of course, the best thing is that the print office gets a print-ready file: it will be cheaper and faster. Sending a PDF is smarter than sending a Scribus file for many reasons:
You're sure that the document you'll get will be the exact print copy of the file you sent because a PDF file is not made to be modified.
You'll avoid some trouble, with fonts for example if the print doesn't have some font that you used, and as we'll see they will all embed into the PDF.
You can add some extra information in your PDF, like printing marks.
PDF is a very commonly used file format and you can easily send you file for proof reading or other reasons and be really confident...