Thanks for that, jleuze.
I read that post yesterday and followed all the instructions: reinstalled, upgraded flash player, changed .htaccess etc, all to no avail. I get exactly the same error.
I checked on one of my older WP installations (a different website, but using the same server) and PDFs upload fine. Maybe I’ll just have to go back to version 2.1 (assuming I can still download it…)
