Why can't I save to a file already existing in acrobat.com?

When I have a file already saved in acrobat.com and work on a similar file on my computer, I cannot save that file in acrobat.com. It gives an error. I have to delete the file in acrobat.com before it will save the changed file. Defeats the purpose of the cloud function, I think.

There is no sync function that I can see either.