If I make a change to a document in acrobat, I cannot save it. An error message always comes up.

I have Adobe Acrobat XI. I can make PDF files very easily, however if I edit a file and try to save, I get the following error message.

"The document could not be saved. The files may be read-only or another user may have it open. Please save the document with a different name or in a different folder."

I have looked, it is not read-only, and I am for sure the only one with this file open.

Any suggestions - this driving me crazy!