This file is in Adobe Acrobat (PDF) format. If you have not installed and configured the Adobe Acrobat Reader on your system, see Help with Printing for instructions. PDFs are designed to be printed out and read, but if you prefer to read them online, you may find it easier if you increase the view size to 125%. Many versions of the free Acrobat Reader do not allow Save. You must instead save the PDF from the PNAS Online page you downloaded it from. PC users: Right-click on the Download link and choose the option that says something like 'Save Link As...'. Mac users should hold the mouse button down on the link to get these same options. Try saving the file to disk before printing rather than opening it 'on the fly.' This requires that you configure your browser to 'Save' rather than 'Launch Application' for the file type 'application/pdf,' and can usually be done in the 'Helper Applications' options. If the file is taking more than 10 minutes to download, see Tips for users experiencing slow response. Source.