Disk images couldn't be opened when downloading .dmg file

Since I upgraded to Yosemite I haven't been able to instal a .dmg file for upgrade of adobe - assumed a corrupt file but it wasn't vital so I ignored it until today when I needed to use the player.

I have run disk verification/permission checks - all ok & I have turned off security & firewalls but, even with deleting and redownloading the files several times after purging the error code still says

'The following disk images couldn't be opened' = reason given 'No such file or directory'

I have also tried to download Opera and Chrome today - the same error occurs.

Running Yosemite on Macbook pro from 2013 - all softwares up to date and problem recent - can anyone help?

Simon