Currently, there isn't any documentation for HAPI PDG integration yet, but a good reference would be the Houdini Engine for Unity plugin. You can take a look at the relevant source files here:
https://github.com/sideeffects/HoudiniEngineForUnity/tree/Houdini17.5/Plugins/HoudiniEngineUnity/Scripts/PDG [
github.com]
Here is a list of the PDG-specific APIs in HAPI:
// Query PDG state:
HAPI_GetPDGGraphContexts
HAPI_GetPDGEvents
HAPI_GetPDGState
// Manage PDG cook:
HAPI_CookPDG
HAPI_DirtyPDGNode
HAPI_PausePDGCook
HAPI_CancelPDGCook
// Manage work items:
HAPI_CreateWorkitem
HAPI_GetWorkitemInfo
HAPI_GetNumWorkitems
HAPI_GetWorkitems
HAPI_CommitWorkitems
// Set work item data:
HAPI_SetWorkitemIntData
HAPI_SetWorkitemFloatData
HAPI_SetWorkitemStringData
// Query work item data:
HAPI_GetWorkitemDataLength
HAPI_GetWorkitemIntData
HAPI_GetWorkitemFloatData
HAPI_GetWorkitemStringData
HAPI_GetWorkitemResultInfo