There are many PDF readers available for many platforms. Some are better known than others, but that may not mean they're the best for all uses. Here is a selection of possible choices.
Foxit Reader
A lightweight and fast PDF reader available for many platforms including Windows and Linux.
Evince
I haven't used it yet, but Evince looks quite good. It's intended for the Gnome desktop environment.
KPDF
The KDE project's PDF viewer is directly based on xpdf. Although not, in my experience, able to handle large files it is quick and reliable with the smaller documents you'll find here.
Xpdf
An open source PDF viewer for the X window system. A good choice for minimal environments such as XFCE.