Unnecessary 'resave'

Hi, I have a problem.

For my studies I download pdf's in acrobat. I highlight text and add comments. When I then save before I close it, it basically performs a 'save as' function with which I have to navigate to the folder in which the file is already located and resave it. I then get the prompt: 'There is already a file with the same name, do you which to overwrite it'. How do I get the program to just save the changes like with any other program. For example, with microdsoft word I just Ctrl + S and it just saves the changes. 

There might be a quick fix I have missed. 

Thanks in advance!