virus – Bizarre PDF: file displayed ok yesterday, today shows only dots and lines

Yesterday my wife sent me a PDF file as an email attachment. I opened it on MacOS and printed it. Today she says the file (on her Windows 10 computer) is “corrupted”. So I tried re-opening the email attachment — and my copy is also corrupted: it displays as boxes, lines and dots. Same thing on my iPhone.

It’s as if the PDF had a built-in Mission Impossible timed self-destruct.

I tried converting it to ps (using pdftops). I’ve looked at it using pdfinfo. Nothing stands out.

Any ideas what might be going on, or what I can try?

Apologies if this is not the appropriate place for the question — a web search didn’t throw up anything similar.