You could try Calibre (at calibre-ebook.com).
It can convert from and to many different formats…
From: CBZ, CBR, CBC, CHM, DJVU, EPUB, FB2, HTML, HTMLZ, LIT, LRF, MOBI, ODT, PDF, PRC, PDB, PML, RB, RTF, SNB, TCR, TXT, TXTZ
To: AZW3, EPUB, FB2, OEB, LIT, LRF, MOBI, HTMLZ, PDB, PML, RB, PDF, RTF, SNB, TCR, TXT, TXTZ
…but not directly from a Scribus file.
It can convert from PDF as you asked but you'll need to take various things into consideration: http://manual.calibre-ebook.com/conversion.html#pdfconversion
However, it's free and open source so installing it and seeing if it works for you shouldn't be too much of a bother. Try it, and if works then great, or if it doesn't just uninstall it.
If this doesn't do what you want I hope another forum member will come up with a better solution.