Sumatra PDF is software which can read PDF files.
PDF Readers are a dime-a-dozen these days because the official Adobe product Adobe Reader has some drawbacks, most importantly that it can be slow and sometimes crashes.
Here is a list we have compiled of the top PDF Viewers which are available. Some of the best are in fact freeware titles. But beware when installing some of the freeware programs, as they may offer toolbars and such.