I'v wrote a plug-in file (X.api) by Acrobat DC SDK. Could u tell me the steps of using MakeKeys.exe and SignPlugin.exe to identify the file and the fees if it's needed?

The fees are by negotiation with Adobe. Prices are secret. You describe your plans in detail and they decide whether to allow it. The top tier of plugins, which have the most checking, are DRM plugins at many tens of thousands of dollars. Annually.

None of this applies if your end users pay for Acrobat.