Take a minute now and check - I'll bet you have at least one PDF on your computer already. It's words and pictures stored in a particular way invented by the Adobe company. That's the same outfit that invented the PostScript language, a language computers can use to tell printers just exactly how to print a page of words and pictures. Later Adobe recognized that people were having trouble sharing documents from one computer screen to another. Things are fine if both people are using the same computer with the same version of the same software.
Sharing is easy.
But plenty of times, both people don't have the same wares. Theoretically the words and pictures are all there in most any format, but in practice the tiny differences or misunderstandings between programs and versions can mean changed colors, missing italics, lost tabs, confused tables, skewed page positions, and worse. Windows, for example, opens them with its Notepad utility program. Or you can open them in most any word processor. One possibility is simply making everyone use the same program, such as Microsoft Word.