PNAS strives to ensure articles are accessible for readers throughout the world by offering article PDFs that are easily suitable for downloading in any environment. In order to accommodate size and content, article PDFs are processed to display images at 200 ppi, which will guarantee quality at print size. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.