Abbyy Cloud OCR has REST API - it may be accessible directly or through client .NET library. You should be able to find out details in Abbyy's documentation. You can try to utilize REST client component or use custom C# code on Pega Robotics side for integration.
However please be aware that Pega Robotics comes with Document OCR component OOTB and it is based on ABBYY engine. Check the following links: