Try disabling: Edit > Preferences > Document > Save Settings > Automatically save document changes...
I had the same problem and managed to find the answer by going through the "Changing settings in Adobe Reader" on the HMRC website. This is on the screen just before you download the Corporation Tax Return you are going to fill in, you need to print out the instructions as there are about 16 pages.
The best way to check to see if you have a problem is to follow the instructions in step 12 to see if you have all the correct trust settings, if one of the settings is missing it will cause this problem. After step 12 they go through and tell you what to do next to get round the fact that one of the trust settings is missing.
It doesn't take long to sort it and touch wood mine is okay now and I can get on with filing corporation tax returns.
Hope I'm not too late with a reply but here you go....
follow the instructions using this link:
if you have adobe v10 they have a diff instruction set)
http://www.hmrc.gov.uk/ct/inline-xbrl/index.htm - link to show both v9+ and v10+
Works very well!