Thank you so much!
Finally, I did not need a File Manager. I just had to check a specific box in Quickoffice settings : « display advanced devices » (my translation).
But I have another question. I am mostly interested in using my phone OFFLINE so I try to save some documents on the intern memory. I was able to find the subway map as a PDF, which I could save easily. But there is another map (bike share map) that does not seem to exist as a PDF that I would like to save on the phone. I found the MAP (interactive map format), I found and saved the APP, but I can t find the way to save the map on the phone in rder to access it OFFLINE.
Is it possible?