The PDF format is somewhat awkward to edit. I've never put much stock in the idea that this is one of its virtues, but for this specific kind of manipulation I can see it might be effective.
Subtle formatting errors make it apparent when recruiters have reordered lists or inserted bullet-points here and there.
They'll try to cover contact details with white rectangles - infamously not great at removing data in a digital document.
Even when it it's not clear from the document itself, I like it when candidates come in with their own copy of the CV, I'm always interested in comparing the difference between that and the one the recruiter provided.
In the last ~4 years I've only sent out PDF copies of my Resume to Recruiters.
A couple of have turned around and asked for it in DOC/DOCX, but a whole bunch more have just copy/pasted it into their own template and sent that to the employer.
I've turned up to interviews and offered the interviewers copies of my resume, and they wave something with the recruiters's logo all over it and not looking at all like what I sent in.
As soon as I'm talking to a candidate 1-on-1 (without the recruiter proxying) I ask them to provided me an original copy of their Resume, because the recruiter may have left out important details (thinking them irrelevant, as recruiters typically aren't tech-savvy) when copy+pasting into their template.
PDFs are pretty trivial to edit anyway.
I suppose you could print out your CV and scan it into a PDF to prevent that - but then there is OCR...
You're right of course that with OCR, and the right typefaces, it's always possible to automate the process of building an editable document from a PDF or printed page.
Also, who would want to lower the perceived visual quality of his resume? And scanning a document, means just that. It will be still readable, yes, but you can see, that it was scanned in.
I had a couple of recruiters ask for a doc/docx. I think I only made one conversion where it was the actual HR from the company using it for keyword matching and I refused to do it for the other recruiters. Still got invited to interviews of companies for those other recruiters.
It might seem harsh, but it's an easy way to weed out lazy recruiters that just want to spam your C.V. to many clients based on a keyword match when they upload it into their automated system and the recruiters that want to take time and even invest into a relationship to build a decent portfolio.