how can i add custom plugins to adobe acrobat reader? basically, i have run the solution file in visual studio and .api file is generated. now how can this be added to adobe acrobat reader when i open adobe software

Duplicate, answered elsewhere (how to buy Acrobat).

No, use Acrobat. Acrobat Reader is not suitable for your development, even if you eventually intend to pay thousands for a Reader plug-in license.

see.. after .api file is generated, i put it under the adobe<plugins folder.

then i opened the acrobat reader DC to see if that plugin was added. but it is not

First test it with Acrobat (paid product) NOT Acrobat Reader. Does it work? If so you can apply to Adobe for a license to make plug-ins for Reader. If they like your idea and accept your application you can pay the fee and receive the key needed to make Reader plug-ins.