If you’re somewhat like me and read alot of books or technical documentation in pdf or djvu format, then I’d like to suggest you checking out the Okular document viewer. It is a part of KDE project, and it has at least two excellent features which make it well worth setting it as the default document viewer.
Review mode
Review mode is a bunch of tools that allow you to make the notes and scribbles inside the document you’re reading. The following picture shows the document with a custom note and review toolbar visible:

The notes themselves are saved transparently and independently from the document you’re viewing (under ~/.kde4/share/apps/okular) and they won’t mess with the originals — unlike e.g Foxit reader for windows, which insists on saving the file with the notes into a new pdf.
Trimmed margins
When reading documents from the monitor, the large margins that look nice when printed tend to get in the way and waste space. This feature removes them, making continuous reading easier, like this, original with margins visible:

And the trimmed margin view:

These little things make working with the pdfs and reading from the screen much more enjoyable. Try it out — it should be easily found in the software repositories of your OS.
P.S The scary pictures come from the free book Introduction to Statistical Thought by Michael Lavine.
P.P.S I would really love to hear about how cool the pdf readers are under mac. While I’m an avid linux user, I’m very interested on how things are done in other systems.
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