How does that stop them? Most 3rd party readers have basic editing capabilities and any unethical recruiter could trivially afford (or unethically obtain) Acrobat Pro.
It just makes it more difficult. Everyone has Microsoft Word, but not everyone has Acrobat Pro, or the other 3rd party tools. If you're really paranoid, you could digitally sign your PDF, but that seems like overkill.
Recruitment companies use automated CV parsing software to extract structured data (contact details, employment/education histories, coded skills based on keywords in your CV, etc, etc) from all kinds of CV formats, including PDF.
From that structured data, they can recreate your CV content in their house style, even if it was originally a PDF.
(My employer is a market leader in this area, although 90%+ CVs I see are Doc files, and we mostly use an automated Word process for stripping contact details from those and adding recruiter's logos.)
Only the smallest recruiters are doing any of this stuff by hand using Word/Acrobat. It's all automated, like a sausage factory.
Anyway, if you make it hard for them to process your CV how they like it, they'll just chuck it. In that case, why bother sending it to recruiters at all? If you need their help, you need to play by their rules.
Right, it's just deterrence. No amount of signing is going to stop a determined and evil recruiter from just retyping your CV with their buzzwords added. Can't imagine that actually happening, but I am reminded of the employer looking for "pink box testing" experience...
You've been leading a sheltered life. The majority of CVs we received via a recruitment agency at Kuju (game developer) had been changed by the agency, and I'm not just talking about removing the contact details.
Some CVs were so mangled that I'm pretty sure they were a copy & paste job from a PDF into MS Word.
My piece of advice: if you ever have an agency apply for jobs on your behalf, insist on seeing the final version of your CV before they send it. They probably mean well, but they frequently end up misrepresenting you because they just don't understand your job. Yes, this also applies to agencies specialised on the tech sector.
