Customers always get corrupted files (file protection)

Hi,

I protect few zip files with aMember v5.3.1.

But when my clients download them, they get a corrupted file (impossible to open).
What I tried:
1. I check my zip file on my computer: they are ok
2. I uploaded through aMember again.. but still, my clients get damaged file
3. I uploaded myself through FTP then added it in aMember.. but still, my clients get damaged file

Where can come from this error ?

Thanks for your help