Okular is a universal document viewer developed by KDE. Okular works on multiple platforms, including but not limited to Linux, Windows, Mac OS X, *BSD, etc.
You can watch for development at the project Home Page.
Its development began as part of Google's Summer of Code program. The description of the project can be found at KDE Developer's Corner.
Okular combines the excellent functionalities of KPDF with the versatility of supporting different kinds of documents, like PDF, Postscript, DjVu, CHM, and others.
The document format handlers page has a chart describing in more detail the supported formats and the features supported in each of them.
As well as supporting many formats, Okular offers such features as text selection, annotation, extraction of files embedded within a document, and many other surprises. Screenshots of Okular in action can be seen here.
You can talk to developers and other users on IRC, irc.freenode.org, channel #okular.
If you are interested in contributing to Okular, please contact the team. Programmers and non-coders are welcome all the same.
Double clicking on either one of the Review tools (F6) allows you to make multiple annotations without having to reactivate the tool after the first time.
How to keep the annotations you made to the PDF file:
By default, Okular saves annotations in the local data directory for each user. Since KDE 4.9, it's optionally possible to store them directly in a PDF file by choosing, so they can be seen in other PDF viewers. Note that this feature requires Poppler 0.20 or newer for regular PDF documents. If the PDF document you are annotating is encrypted, this feature requires Poppler 0.22 or newer.