I had minecraft on my computer before but was playing through a file in evernote (if that makes sense. I saved the application from my iPod into evernote then opened it on my computer. I'm not sure how, but it worked :P) now I'm trying to download the application from minecraft.net but it won't download correctly. The file is supposed to be 270 kb but in downloads it says its only 149 kb. When I try to open the application, a pop up comes up and says "minecraft is damaged and can't be opened. You should move it to the trash" 
Everything is current on my computer. Any ideas on what to do?