Well, there is a case for PDFs in a .zip, but it's when you're e.g. downloading some set of documents from your e-government site, and it's like 4 PDFs, all cryptographically signed: so you get a .zip with 4 PDFs in it and 4 .sig files with matching names.
But in emails? Just attach however many PDFs you need and send, they don't really compress anyway; and I think most web-mail fronts actually allow you to download all the attachments as a single .zip — but obviously those .zip-files are not maliciously crafted (I hope, at least).
Also, now that I think of it, forwarding the PDF you've extracted and visually reviewed instead of the original .zip-file would defeat this attack (unless, of course, it's the PDF file that's schizophrenic).
But in emails? Just attach however many PDFs you need and send, they don't really compress anyway; and I think most web-mail fronts actually allow you to download all the attachments as a single .zip — but obviously those .zip-files are not maliciously crafted (I hope, at least).
Also, now that I think of it, forwarding the PDF you've extracted and visually reviewed instead of the original .zip-file would defeat this attack (unless, of course, it's the PDF file that's schizophrenic).