Adobe Acrobat Reader has built-in accessibility support for reading out PDFs. The current version 7 can be installed under Linux. If you don't already have the reader, navigate to the product's download section and choose between an .rpm file (41.9 MB) and a tarball (42.1 MB). Installing the RPM package should be as simple as
rpm ivh AdobeReader_enu-7.0.8-1.i386.rpm. If you are on a non-RPM-based system, get the tarball, untar it, and run the included INSTALL script.
Once the reader has been installed, navigate to the View -> Read Out Loud menu. If the options are greyed out, you'll need to hook it up to a speech synthesis system.
Festival is a free text-to-speech (TTS) system written in C++ that'll work. If you're on a Debian-based system,
apt-get install festival should do the trick. Others should follow the installation instructions on Festival's Web site.
In addition to Festival, you need the gnome-speech library, which provides an interface for applications to convert text to speech. Ubuntu Dapper Drake has this pre-installed. You can also manually install the API. Once that's done, try its
test-speech tool to select and test the TTS synthesizers installed on your system.
That's all. Launch the reader again, open a PDF, visit the Read Out Loud menu, and let go of the mouse. Don't forget those headphones if you're in an office, though!