Word document and PDF are the most used document formats in a formal or informal settings.
In recent versions of Microsoft Office, there is a very convenient way to either export or convert any Microsoft Office documents into a PDF.
PDF is a great type of file to share between different operating systems, because there are typically no conversion problem.
You can really count on it to appear the way you intend. It helps prevent editing.
It typically has smaller file sizes, and for official communications, many businesses and organisations prefer the PDF standard.