Acrobat SDK pricing

Hello!

I'm a beginner and I'm sorry if this question sounds stupid. I researched quite a bit and I'm still not sure about the Acrobat SDK pricing. To be more precise, is it free to use AxInterop.AcroPDFLib.dll and Interop.AcroPDFLib.dll generated as com references in visual studio? We are going to run the project only on client's machines having Adobe Acrobat DC Pro installed. 

Is there any cost for it?

Thank you!