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.
Well I guess it's gone now. Windows malware is far more common hence Mac users will probably encounter more detections from malware targeted at the Windows OS. Windows malware is generally just a file and though it can't run on a Mac it could be downloaded (through email or a browser) or transferred from a USB pen drive or network share. If it's through your mail application the file can be in your mailbox folder on the local drive and then detected from there - just by syncing your mail with the SMTP/POP server.
Let me know if you have any other questions, or mark a post as the accepted solution if answered.