UnsupportedDocumentException: Request has unsupported document format When sending in a pdf with more than one page.

The synchronous DetectText API supports only 1-page PDF documents no larger than 10MB (source). These limits are in place because the API is synchronous and there is an expectation that the result will be returned quickly. A multi-page PDF document takes longer to process and can only be done with the asynchronous StartDocumentTextDetection API.