Which is the best pdf reader?

Since you are new to PDF format, here's a brief introduction to the format first. The Portable Document Format (PDF) is a popular format to publish formatted text and documents. There are several different versions of it, some qualifying as an Open Standard, some certified by ISO and some encumbered by software patents. Portable Document Format (PDF) is an open standard for document exchange. This file format created by Adobe Systems in 1993 is used for representing documents in a manner independent of application software, hardware, and operating systems. Each PDF file encapsulates a complete description of a fixed-layout flat document, including the text, fonts, graphics, and other information needed to display it.

Now there are a number of tools available for viewing PDF files on your computer and most of them are free to use. Though you didn't specify which operating system you are using, from your query it is pretty obvious that you are using a version of Windows. Here's a few PDF readers available free of cost for Windows,

Among these software, I really recommend Sumatra PDF because it's very small, open source and can also read DjVu files.