If you've ever worked with a third-party recruiter (aka headhunter), you may have been asked to provide your resume specifically in Microsoft Word format instead of PDF. Perhaps you spent hours formatting your resume with the intent of using it in PDF (not Word), so you didn't pay much attention to how it looked in Word.
When recruiters ask for your resume in Word format, it can be for a few different reasons.
1. Adding Logos and Branding - When agency recruiters send a resume to their clients (hiring companies), they want to make sure that the client is fully aware which agency sent the resume. This concern dates back to when agencies sent resumes via fax, and there might be a pile of resumes on the fax machine. In order to ensure that the reader knew which agency to credit (and perhaps pay if a hire is made), agencies would put their logo and recruiter contact information on the top.
2. Editing or "Blinding" - Some recruiters may want to make some small (or large) edits to your resume in order to clean it up or perhaps to meet some requirement the client has. This practice can be a bit controversial, as some recruiters have been known to change the content in substantive ways that may exaggerate the candidate's qualifications or experience. This practice may be done without the candidate's consent, which can lead to some surprises during interviews. "Blinding" a resume is when a company will strip out identifying information, such as name, contact information, employer names, education and schools, etc. This might be done for a variety of reasons. A company may request a blinded resume to try and prevent bias from their resume screeners. Sometimes a recruiter may blind the resume of an impressive candidate and send it to a company in order to try and gain that company's business.
3. Importing - The recruiter may need to import your resume content to an ATS system, database, or some other system. Copying and pasting from Word is much easier than PDF in most situations.
4. Client Requirements - Some hiring companies may have a standard for all resumes to be submitted in Word format for consistency, so the agency recruiter is just following orders.
To prevent any issues, always ask recruiters to ask permission before editing your resume's content in any way and be sure to get final approval on material changes to your content before it's submitted to a hiring company. You can also ask agencies not to send your resume anywhere without your prior approval. Some recruiters will be better about obeying these requests than others.