PDFs, or “Portable Document Format” are electronic documents. PDF is a standard that will always display a document in the same shape and size. Not everyone has MS Word, Word Perfect. Saving documents as PDF, ensures that whomever you send your document to, will be able to open them.
You’ve probably seen or opened a PDF and may not even know it. Historically, Adobe has ruled this format (as they did start it) with their free Reader program.