Acrobat bad parameter

This isn't complex. I created a PDF file, which is what I bought ACrobat to be able to do,

laid out text in it, formatted it, saved it as a pdf file.

Go back to edit three lines of it, try to save it, and get bad parameter error on save as. First off,

on just save I'm told someone else has the document open (not). The file may be read-only.

Really? When did I set that? Why would an editable PDF file suddently turn read-only?

I can't even begin to explain how unacceptable this is.