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


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

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

Any ideas?