What most of us don't realize is that PDF is a standard, and like any standard, it's specifications are widely published. As such several programs are available, most of which are free and some at least cheaper than Adobe's offering, that allow us to author/edit PDFs. Yoav Ezer has compiled a list of more than 50 such tools. Have a look.
[This is part of a series of posts, titled Useful Links, which shall feature "essential" online tools, discussions, blogs, essays and references that we find over the Internet.]