It's in the repos as "acroread" or download it directly from Adobe
Frankly, I recommend against the Acrobat Reader. It's bloatware.
Gnome on Lenny should already include the Gnome app for reading PDFs and maybe even xpdf (it's been a couple of years since I used Debian).
My choice for a PDF reader is Okular
, which should also be in the Debian repos (I know it's in the Ubuntu repos); it should install and run just fine, even though it is a KDE app.
On Windows, I use Foxit
rather than the Acrobat Reader. There's also a Foxit for Linux which I've heard good things about.