Have you ever opened a document made on a Mac in Windows, or visa versa, and seen a bunch of strange characters and unreadable text instead of a beautiful document?
That's why Adobe created the pdf format. Adobe came up with the idea in 1991 and released the format in 1993. The Portable Document Format allows you to transfer documents from one type of computer to another and maintain the fonts and images, so the document looks the same. Since the web became popular, pdf is the most common format to transfer documents to and from websites.
Adobe's strategy in the beginning was to give the readers of the pdf format away for free and to charge for Acrobat, the program used to create pdf files. So, anyone could read a pdf, but only people who paid for the software could create a pdf or edit a pdf.
Adobe started giving away the specifications to the format in 2001 and other companies created programs that could write pdf files. Now, most word processors have the ability to save a file as a pdf. The best way to make changes to existing pdf files is still Adobe's Acrobat program.