I am using Adobe SDK to develop .api. Using sample code - basicplugin project. It can be built successfully. I put .api into adobe plugins directory. Reopen Acrobat and it can show menu. But it will show “invalid plugin detected. Adobe Reader will quit". I don't know if it needs to be enable by using tools Makekeys and singpluing to enable .api.
For the use in Acrobat Reader you must enable the plugin.
Enable with MakeKeys, as described in the documentation. But first, you must apply for a license from Adobe, they will review your application, and might (or might not) allow it, then quote you a fee - up to $25,000 pa for some types of plug-in. You will also need Acrobat (paid for) to develop and test the plug-in, even if writing a Reader plug-in/.
Also: Adobe will NOT allow you to make a plug-in for an obsolete version of Reader. You must use Reader DC (as well as makekeys, licensing etc etc.)
I have applied the public key, it is fine now.