Does anyone know how to open a PDF attachment in GMail.

You usually download it and then open it with Adobe Viewer.

You need a PDF viewer installed on your computer. Sumatra is a good lightweight viewer: Free PDF Reader - Sumatra PDF (http://blog.kowalczyk.info/software/sumatrapdf/free-pdf-reader.html)

You need a PDF reader to open that PDF you received, you can use Adobe Acrobat (http://www.adobe.com/products/reader.html), or you can have a try with PDF Reader for Mac (http://www.cisdem.com/pdf-manager-ultimate-mac.html), Foxit Reader (http://www.foxitsoftware.com/Secure_PDF_Reader/addons.php)