Location: Tottenham, Ontario - far above the centre of the universe
I’m currently working through a similar problem with a client.
A possible work-around is to “print” the file and select “save as PDF” instead of sending to an actual printer, this avoids the original application creating the file.
It doesn’t always work, another tactic is to compress the file (Zip) when sending as an attachment. I’ve had problems when the security software on a clients server would chew up the attachment and leave it corrupted. Zipping it seemed to calm things down.
In order to "Print" the file, one must be able to open it. Even choosing it in the Finder and selecting "Print" will result in an attempt to open - and this is where we're stuck. Nothing we have will open it.
I opened it with TextWrangler to check the contents, and it appears to be a valid PDF, just not one that his Mac (or mine) is willing to deal with.
Zipping when sending as an attachment may well do the trick - but the problem is that there's no consistency as to the source of the file. PDFs from various sources become "damaged". So I suspect it's his MacBook Air, and possibly Thunderbird, causing the problem during reception.