Free and open-source graphical interface for pdftk (PDF Toolkit), a command-line tool used to manipulate PDF files.
Create, edit, convert, and view PDF documents.
Program to create non-Word or Excel PDFs.
Adobe Acrobat Reader allows you to view, comment, fill out, and sign PDFs on multiple platforms.

Utility to extract and merge pages from PDF files.
Free software that allows you to merge multiple pages of PDF documents into a single page.