
Foxit Reader for Pocket PC is a slick utility with a small footprint (1.4MB) that enables you to read PDF documents on your handheld. It's still in beta and as such doesn't support advanced features such as tagged PDF documents, JBIG2 or JPEG2000 encoded images, ICC color profiles, nor does it offer any reflow options to adjust to the small screen.
A free evaluation version is available
here.
Alternatives:
RepliGo,
PocketXpdf,
Adobe Reader for Pocket PC,
PalmPDF (for Palm OS)