The GetDoc_1 or 'Document Retrieval' API is a HTTP GET call. You need to include the application ID and cookie in the request. In the response you recieve a URL redirected to the PDF document. you need to use the URL to download the document. Screenshot attached from TRKD support portal.
For more details you can also refer to 'Document Retreival' chapter within TRKD API Programmers guide (TRKD API DEVELOPERS GUIDE).