Adobe Acrobat Reader allows you to view, comment, fill out, and sign PDFs on multiple platforms.
Free and open-source graphical interface for pdftk (PDF Toolkit), a command-line tool used to manipulate PDF files.

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.
Free software to quickly and efficiently view, create, and edit PDF files.
Small utility that allows you to easily split pages of PDF files.