Word has no control over how a PDF gets re-scaled (or not) by any PDF reader. That functionality, if it existed, would be at the discretion of the reader software and would require PDFs to provide for the inclusion of metadata that could be used to that end. I know of no such capability in PDFs - and there's no evidence of such a security setting in Adobe Acrobat (open any PDF and check the Security options via File>Properties>Security for yourself). You can't even prevent a Word document from being re-scaled for printing...

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]

OK. Thanks. So if I understand you correctly it's not currently possible because neither Word nor PDF have a never-print-in-any-way-except... parameter and they'd both need to have it before the chain of Word -> PDF -> hardcopy was secure against unofficial page size changes.

Sounds like the place to start would be getting PDFs to support it, so the best place to begin would be by writing to Adobe.