One school of thought says pdf-s are not as machine readable as word documents. Some “online job search gurus” to suggest avoiding PDFs so that machine can parse keywords from MS Word better. Unless a recruiter/website specifies a type of document the candidate can only guess what the recruiting system wants.
Sending a document via e-mail warrants a properly formatted document. So may be a fraction of the resumes send into the resume black-hole needs to be in PDF. However, some recruiters are generous enough to give feedbacks. Seeking advice and suggestions on resume is aided by the ease in which the mentor can comment and keep tracking changes in MS Word. An open source enthusiast may frown. I have not seen a more popular word processor than MS Word. The most popular may not be the best. But it is worth if it works.