If you downloaded a file and are not confident it is safe, do not open it, run it, or enable macros or other active content.
Do not send the file to other people simply to ask whether they think it is safe.
If the file came from a suspicious message, keep the original message and sender details as evidence.
If you already opened or ran the file, stop using the affected device for sensitive activity and disconnect it from the network if you are seeing unusual behaviour.
Run your device's trusted security software and follow its recommendations. Avoid downloading random 'cleaner' or 'virus removal' programs from links in pop-ups or suspicious messages.
If you entered passwords after opening the file, change those passwords from a known-clean device.
If you are seeing signs of compromise, use the hacked/computer-infected guidance rather than continuing to investigate the suspicious file yourself.