.pdf = PDF's are designed to be universally readable. To read "pdf" files you need may need Acrobat Reader. It is installed on most computers by default. This is available as a free download from http://get.adobe.com/reader/ More info on.pdf and what to do with it? CLICK HERE for the answer.
.doc = Microsoft Word document. You may need this particular software on your computer to read this type of file.