Acrobat Reader refuses to close

Hello

I download some PDF files from various locations. For some sources (per example the scanner of my brother, or the web-site of my apartment manager), I have always the message "voulez-vous enregistrer les modifications apportées à xx.pdf avant de fermer ?" or "Do you want to save changes made to xx.pdf before closing ?" (automatic translation).

Obviouly the file is not modified.

What is the cause ?

How to pass over this question ?

Thank you for your responses, I searched by Google by never find any response.