Install any software-viewer of documents in this format if your operating system is the pdf file extension is not mapped to any application. If you plan only to read, not to create such documents, it is best to use the program from the company Adobe Systems developed this format. The application is called Adobe Reader and is free, and download the most recent version is available on the website of the company http://get.adobe.com/reader/. After you download and install any running you a file with the extension pdf will be transmitted to operating system that program. In addition, you will be able to view these documents, for example, directly in the browser if they will meet you on the Internet.
Use the online services if you want to work with this file format without installing the computer software. Such services usually require registration, and after that allow how to view and create pdf documents. These include, for example, Google Docs, FreePDF.org, acrobat.com, pdfescape.com and others.
Map the extension pdf program to view these files if it is installed in the computer, but the operating system does not transfer files of this format. To do this, click the pdf file, right-click, in the popup context menu, expand the "Open with" and click on "Choose program". In the applications list in the window that opens select the right one and if it is not, then click the "Browse" button and find the executable file for the program on your computer, and then click Open. By choosing the application, check the box "always use this program to open this kind of file" and click "OK".