Until recently I had no problem sending pdf files in outlook express but now the poor recipient gets about 20 emails supposedly with the pdf I sent, but none of them will open it.
I can send the same file from my online Sky email without any problem
I can send and receive other attachments in express - no problem
It may well be the update problem. As you say you were able to to it previously.
Have a look in Control Panel > Add/Remove Programs > Make sure you have the 'Show updates' box in the top right checked > Scroll to the latest update at the bottom. They should be in installation order. Highlight the one you think maybe the problem > Remove.