Help Please about downloads

I did a test purchase of one of my ebooks and the download worked but when I tried unzipping it with winzip it stated

cannot open file:it does not appear to be a valid archive.
If you downloaded this file, try downloading this file again.

I don't have any ideas to what could be wrong. I am on a linux server and I have the permission set to 777 under the downloads folder and download by redirect is on. I don't know what to do. Can someone help me figure out what is wrong or can someone share their experience with a problem like this to possibly help me. :(

I fixed it. Yes yes yes. Even though no one helped me I'm going to tell how I fixed it if it can save someone else a two day headache. Under my /downloads file in file manager I have all my ebooks saved to /ebooks. I changed the /ebooks permission to 777 and all of my zip files in that folder I had to change the permission to 666. If you are having this same problem try this, it might just work. I am so happy now. I can finally go live with my store. YES!!!!!!!!!!

Every webserver's configuration is different.

YOUR hosting company's unique server configuration probably prohibits you from being allowed to use 777 permission levels. Keep that in mind for other things you may do on your server.