Subject: About Stream

Hello. I trying this code:
PDDocument documentpdf = PDDocument.load(new File("file.pdf"));
PDPage page = documentpdf.getPage(0);
Iterator<PDStream> contentStreams = page.getContentStreams();
   PDStream next =;
COSStream cosObject = next.getCOSObject();
So i get stream from PDF (Returns the contents of the stream as a PDF "text
string".) .
What meanin this content? I try to parse it for some image settings. Where
i can read about format of tis content? Maybe in PDF specs?