MarketPress PDF downloads corrupt on live site, but not on dev site

Hi

When users download PDFs from my site they are coming out corrupt (see attached for acrobat error message)

I can't pinpoint exactly when this started happening, but it was around 2 weeks ago.

I'm using WP 3.9.2 and MarketPress 2.9.5.2

On my test site, with the same Wordpress and plugins, the downloads work correctly.

Any ideas?

Thanks
Pete