Acrobat can't save edited file

When I have made changes in our documentation I make a pdf with all changes marked in red (I make the changes in FrameMaker). As an extra thing, I open the pdf and use the text tool to write a short summary on page 1. Why don't I do that in FM? Because all the red text I can easily search and replace with black, but it's easy to forget to remove the text on page 1.

So, I open the pdf, use the text tool and write. For the last couple of months there has been a problem with saving the file after that (I have done it this way for at least 15 years!). I want to write over the existing file, so I just press Save. But after a couple of seconds Acrobat tells me that it can't save to that file name and that I have to choose another file name or another place to save it. OK, I could live with that, but now my file has a gibberish name (ends with .tmp) and I have to be really careful when I re-name it to the correct file name, as ours are really similar to each other.

Why does this happen? It's not always, but say 85 % of the time. It doesn't help if I press Save as and point to the correct folder (where the original file is). I could press Save as and for instance save on my desktop but I always forget. And then the file is re-named and I have to change it anyway.

Does anybody have an idea of why this is happening and how to fix it?