Calling OCR

The REST API is not as comprehensive as our COM API.  If you need to do something that is not exposed then I would consider creating a Vault Extension Method (in either VBScript or VAF) that uses the COM API to achieve what you need, and then call that via the REST API.

Another option is to use the COM API from within a workflow state, and use the REST API to move an object into the workflow state that then triggers the code.

Is the result of OCR just to create a searchable pdf? If I created an object and attached an already searchable pdf, would that be the same result ???

Broadly, yes.  It's for scenarios where you have a PDF without a text layer and want to run the OCR engine to create that text layer for indexing and the like.