I use the Adobe Reader for Pocket PC 2002 on my Axim. It's a tricky program; it reads PDFs and adds markers so that it can be read more easily on the pocket format. If your PDF can't take the markers (i.e. if it doesn't have legible, easily defined columns of text) then you can still look at it as if you're holding a deck of cards up on an 8 1/2 x 11 sheet of paper; you've got to do a lot of scrolling and it's very impractical for lengthy reading.
I am not familiar with what options are available for Palm-based products.