You must have a reader software to open a PDF file.
PDF Readers for Windows
Adobe Reader is widely used and free application that you can use to read PDF files. It can open standard PDF files and PDF portfolio with multimedia. It is the most powerful PDF reader but slower and larger than the others. Adobe Reader has both the Windows version and the Mac version.
Foxit Reader is a faster and lighter application that can view PDF files. It is easy-to-use, the latest Foxit Reader has commenting tools, you can highlight, underline, add comments or some other basic editing functions. It is available for all windows based operating systems.
PDF Readers for Mac OS X
Preview is a built-in application that makes it easy to read PDF and popular image files on Mac OS. It has basic editing capabilities such as resizing, rotating, annotating etc. It is the best PDF reader for Mac OS.
Lighten PDF Converter apps also have the built-in PDF reader, you can easily read a batch of PDF files.