Antivirus deleted it probably, add %localappdata%/Plutonium to exclusions. And if antivirus deleted it, it will most likely be in quarantine section, just restore it.
In order to get a certificate to be a recognized app you need to pay a large amount of money to a company to certify your program. We do not have the money to do this as it's a community project. I hope you understand. But if my word means anything then yes the download is safe.
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