Unable to overwrite a PDF via save but it will do it when i click save as

I have open an already saved PDF. i open it then adds a comment then click the X button to close/ save. it will ask me if i want to save. I click yes, select the same location and click on the same pdf  click yes when it checks i want to overwrite it.  it then gives me an error saying The document could not be saved. Cannot save to this filename. Please save the document with a different name or in a different folder.  Why? it would do it a few weeks ago. but If i go to save vide file-save as and select the same pdf to overwrite it works