I have been trying to attach a 4MB Adobe pdf file to an Outlook email and keep getting the message "there is an error attaching this file"

The file opens ok on my computer.

I am using Windows 7 with the IE10 browser.

I have tried re-creating the file and also clearing the browsers cache, both had no effect. I also have latest version of Silverlight.

I also tried loading another pdf file which I created the previous day and that loaded OK.

Now the really puzzling thing is I also tried attaching it whilst accessing Outlook through the Chrome browser (on the same computer) and it worked fine! I could also then open the file whilst accessing Outlook through IE10, but it still wouldn't attach it to another email whilst using the IE10 browser.

So I could switch to the Chrome browser to solve the problem, but as a creature of habit prefer IE10.

What's causing the problem?