PDF documents are files that contain all the required elements to be viewed or accessed without their original creator application.  This is useful for sending completed documents to others, as they don't need to have the same system setup that you have.  PDF is the shorthand for "Portable Document Format".  Adobe Systems created the PDF format, but now it's considered largely and open standard.  PDF's can usually be opened by web browsers, by alternate helper applications like Preview on a Mac, and Explorer on Windows.  They can be sent up to Google Drive, or opened with Adobe Acrobat.  In some cases a PDF may be created with fields that are modifyable. You may have to use Acrobat to access these features.