Let’s say you’re applying for a job, and the recruiter/job posting asks you to send a resume/CV to their email address.
No file format is specified.
In this case, always make sure that the documents you are sending are in .doc format. You can do this by putting your resume in a Microsoft Word document, then clicking File -> Save As, opening the “Save as type:” drop-down box and choosing “Word 97-2003 document.”
Computers at your workplace might not have newer versions of Microsoft Word installed, and thus won’t be able to open .docx files (newer Microsoft Word files).
Computers at your workplace might not have any Adobe products such as Acrobat or Reader installed, and thus won’t be able to open .pdf files.
But it’s unlikely that they won’t even have Microsoft Word installed.
So if you want even the HR people at offices that couldn’t be bothered to install software on their computers to read your resume, .doc is the safest way to go.