Dealing with files on ipad

i’m New to iPad and can’t figure out how to save and use files on the device. I’m using an iPad Pro ios11.

someone sends me a file on email or via wetransfer... when I download, I click the arrow top right and select save to Dropbox... the file appears with a space ship / ufo type symbol and I am unable to open it...

Can anyone direct me to how to deal with files on the iPad??