PDFsam, a free, open source, multi-platform software designed to split, merge, extract pages, mix and rotate PDF files.
Merge
Merge PDF files, select the pages, merge bookmarks and interactive forms
Split
Split a PDF file at given page numbers, at given bookmarks level or in files of a given size
Extract from PDF
Extract pages from PDF files
Rotate
Rotate PDF files, every page or just the selected pages
Mix
Merge PDF files together taking pages alternatively from one and the other. The perfect tool if you have a single-sided scanner
Freeware
Download: PDFsam 4.2.2