After I moved the epub/pdf files from an external storage to be stored directly on the website 10 days ago, I got more than 600 visitors and nearly 300 downloads.
The problem is that I got two reports of download links not working and displaying an error 403. I think it's a pretty alarming percentage for the number of visitor, reduced by those who would take the time to hit the contact button.
I did some modification for the possible cause (for those who knows, I changed the chmod) and hope it will correct it. Now, if it don't, please contact me and I will try to continue to correct that.
For the time being, I will entertain myself by doing errors page.
Edit : Thank you Kareljuh for finding the problem !