DLLs in Qt official prebuilt are signed with Qt official certificate, but I compiled Qt opensource myself, I'm wondering how can I sign these self-compiled DLLs? Is it legal to sign these DLLs with my own certificate and distribute them?
You can obtain a code signing certificate from a certificate authority (which will cost you a bit of cash, but it's not too bad) and then sign your custom compiled files with that certificate. As far as I know that's perfectly legal (but I'm not a lawyer, so don't take my word for it - ask a real lawyer if it's important to you).