how to sign a pdf with a digital certificate on ipad pro

YES !
This feature should be available considering many people now use IPads as their main work device. 

I bought a $2,000 12.9" iPad Pro , for work , and it's useless because I can't do certificate signatures.  

Hi Eduardm,

As BerndAlheit said above, it is not possible to sign a PDF with a digital certificate on the mobile application.

The mobile application does not have all the features which are available on a desktop application.

There is a limitation for the applications on the mobile device.

You may refer this help document Acrobat Reader Help for iOS: Creating Signatures and Signing to add signatures to PDF in Adobe Acrobat Reader mobile application.

Hope the information helps.

Regards,

Meenakshi