Okular

Overview
Okular is free open-source software for viewing and annotating PDF files in KDE 4. It appears to be the successor to KPDF.

Cons
Inexplicably, there does not seem to be any way to print a document's annotations, negating much of the potential usefulness of this tool (e.g. filling out forms). The annotations can be saved in Okular's native file format, but they are not included if the file is re-saved as a PDF.

Links

 * project site