When i recieve emails why carnt i open attached documents plz help x x x?

hi there im looking for a job at the moment and ive been sending alot of emails to people and when i receive an email from a job vacancy alot of them have attached document files (which im thinking must be application forms ) so i can download them which i press the button to download and then it says open so i save it then i click on it and nothing happens why cant i see these documents and why dont they open please help otherwise i wont be able to apply for these jobs thanks leanne x x x