It is possible to use the Clustered search web service to search in the content of a document.
Example
The following example looks for api in a publication with an ID of ENQRHyDq~oYAqQn5tJAjKA.
{
"query": "api",
"filters": [
{
"negative": false,
"key": "ft:publicationId",
"values": [
"ENQRHyDq~oYAqQn5tJAjKA"
]
}
]
}
It only returns results from the content of the publication.