Skim review

With Preview, Mac OS X is already blessed with a very capable and useful tool for reading PDF and image files.

Consequently, you might be surprised that anyone has taken the time to develop a competing PDF viewer; but they have, and the early results are impressive. Not only is Skim able to offer all of Preview's ease of use, it also brings additional annotation features that make reviewing PDF-based documents much simpler.

Much of Skim's interface is instantly recognisable. It uses the same navigation tools as Preview (the Up, Down, Forward, Back, Zoom, and Jump to Page controls look identical on the two apps' toolbars), so the learning curve is negligible. It cannot open images files, only PDF documents - but the developers do make it clear that Skim is solely designed to 'help you read and annotate scientific papers in PDF'.

And it does a good job of that. Many features have been added simply because they improve on-screen reading. First is the Reading Bar, a configurable overlay that darkens the rest of the page, except for the line you're reading. It can be moved up and down by holding down Option and hitting the arrow keys, or simply dragged into position. Second are two related modes, Full-screen and Presentation. Both expand the document window to fill the screen, masking your desktop, Menu Bar and Dock.

Full-screen mode lets you really fill the screen with zoomed text, making reading a pleasure. It can be combined with other features, such as the Reading Bar and adding or editing notes. Presentation mode is slightly different, forcing each page to be displayed in full. It could be easily used as a lightweight alternative to Keynote or PowerPoint, as the name implies.

Notes and annotations come in a variety of types, from simple text 'sticky notes' to the more formal addition of highlights, coloured boxes or circles.

Since some of this annotation data might be lost when exchanging used files by email, it's possible to export a plain text file (with a .skim extension) that contains all the markup data. Give it the same name as the original PDF and send both files - if your recipient opens them in Skim, they will be re-combined to display the correct notes. Note that Preview in Mac OS X 10.4 gained many similar annotation tools.

Skim is free, open source and amazingly young, still only at version 0.2. Nonetheless, it already offers real benefits for anyone who has to spend a lot of time with PDFs.