Trouble combining PDFs with Adobe Acrobat 8.

Our accountant was preparing for audit next week when she had trouble combining PDFs with Adobe Acrobat 8. She gets the following error messages:
"Error (file skipped)"
"No PDF file was created because Acrobat encountered an unidentified error".
I tried to look for answers through Adobe chat support. But they don’t help with technical problems through chat. I also tried searching the internet, but also didn't find any fix for my problem. I hope someone in here can help me.
Thanks
