Free and open-source graphical interface for pdftk (PDF Toolkit), a command-line tool used to manipulate PDF files.
Free software to quickly and efficiently view, create, and edit PDF files.
Advanced PDF editing solution that offers a wide range of features.
Software that allows you to organize, edit, and manage pages of PDF documents.

Utility to extract and merge pages from PDF files.
Adobe Acrobat Reader allows you to view, comment, fill out, and sign PDFs on multiple platforms.