As a general rule you are allowed to republish free articles on your blog, as long as you put a reference link in your content.
For example, you can go to ezinearticles.com, search for the article you need using your main keyword, and then republish it on your blog, making sure you link to the original article source.
When you use PLR articles, you are free to rebrand and reuse them to your liking, without the need to refer to the source of the content.
It is important to know, though, that copying and pasting free articles as they are to your blog, won’t probably help you with Google search rankings. Google search prefers ranking blogs with original content against ones that are only copying or curating content as is.