Adobe Acrobat XI

I suddenly am unable to save a pdf file that I've made changes to. I get an error "The document could not be saved. The file may be read-only, or another user may have it open. Please save with another name."

I've downloaded the most recent update, but did not solve problems.

There was a 2010 discussion on this topic that suggested to looked in preferences under General to check a box to Enable (can't remember rest), but I don't have this option.

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