Troj/DocDrop-AU is a Windows-only Trojan. It can't hurt your Mac. Sophos Anti-Virus for Mac detects all know Windows malware so that's why you got the alert.
Though there was no threat to your Mac if you didn't have Sophos installed you probably wouldn't have known about it and it could still be on your computer. It could then have been transferred to a Windows computer on a USB pen drive etc.
Troj/DocDrop-AU is a Windows-only Trojan. It can't hurt your Mac. Sophos Anti-Virus for Mac detects all know Windows malware so that's why you got the alert.
Though there was no threat to your Mac if you didn't have Sophos installed you probably wouldn't have known about it and it could still be on your computer. It could then have been transferred to a Windows computer on a USB pen drive etc.