From a NLM Technical Bulletin Post by David Lipman:
The most common way that a scientific paper appears on the Web is as a single long page that you read by scrolling through vertically. Without the landmarks of multiple columns and separate pages, however, once you scroll back to look at, for example, an earlier figure, you often have to hunt around for the spot where you stopped reading.
Another common presentation format is a PDF, which displays the pages on your screen exactly as they appeared, or would appear, on the printed page. If your display is large enough, you might get one or even two pages on the screen. With smaller screens however, such as a laptop or tablet, when you shrink the page to fit the screen the text is too small to read. Zooming in so that the words are readable means you are back to the problem of scrolling around through the article. Either way, it can be difficult to keep your train of thought as you work through the paper. These problems are compounded when you try to read through an article on one of the new tablets with a 7-inch screen.
To address these problems, NCBI has developed a new presentation style that we call PubReader, an easier way to use your Web browser to read articles in PubMed Central on your desktop, laptop, or tablet computer. Like a printed paper, PubReader breaks an article into multiple columns and pages to improve readability and navigation. PubReader can expand a page to whatever fits on your screen — with multiple columns on a desktop monitor or a single column page on a small tablet. It will even switch to two columns if you rotate the tablet to a landscape view. When you adjust the font size or change the size of the browser window, page boundaries and columns are adjusted automatically adjusted.
Learn More About PubReader
Links to Open Articles in PubReader are Found Next To PMC Articles in PubMed Search Results.
At the moment, PubMed Reader only works with most recent versions of some browsers on some platforms. Here’s a complete list.
PubMed Reader Technology