I am far from being experienced in troubleshooting Scribus but since you write that your need is "very urgent" I offer the checklist I would run through.
“multipage document” is too vague as a description.
What operating system in use(Windows, Linux, Mac)?
What size ram?
How many pages in document?
What mix of text and images?
What language, fonts in use?
What is the size of your *.sla file?
Have you tried Preview?
Have you tried Print Preview?
You can select profile:
PDF 1.3, 1.4, 1.5, X1a, X3, X4, Postscript
Can you try opening *.sla file in XML Copy Editor
http://xml-copy-editor.sourceforge.net In XML Copy Editor validate the document (see buttons in top menu bar).
You have the option to Export as EPS, PDF, Image, SVG, EPS.
Have you tried exporting as other file type?
Here is a similar thread for reference.
http://forums.scribus.net/index.php/topic,2613.msg12090.html#msg12090Read the last post in that thread and try exporting to a different folder.
Have you ever previously exported a Scribus file as PDF or is this a first?