What does it mean and how to I get out / avoid / fix it?
There is corruption in the pdf. If this prevents you from opening the pdf, you are so out of luck. If you can open the file you can try to delete some annotations contained in the pdf file.
Thanks for replying but how can I prevent this from happening again? The source came from a client. Could it be that she got a bad installation of Acrobat (PC version)? I don't have this problem on my Mac version. Thanks!