There are many advantages to using PDF files in the application process. PDF files can be considered as a kind of "standardization" of documents. This ensures in almost all areas that your sent file can be opened and read by the recipient. Thus, there is no binding to a particular operating system or version of Word. If you use a different version of Word than the recipient, the formatting of your document could be changed, the readability could deteriorate significantly, and it could look unprofessional. Often the size of application files is also limited. Here, a PDF file offers advantages because it is smaller. Furthermore, the PDF format offers the advantage of combining all application documents in one file, if desired.
As a general rule, if the job posting does not specify a format or you are unsure, ask the recipient if they prefer a certain file format.