heres the basic idea to get to the annots CGPDFDictionary for each page atleast. after that you should be able to figure it out with help from the PDF spec from Adobe.
1.) get the CGPDFDocumentRef.
2.) get each page.
3.) on each page, use CGPDFDictionaryGetArray(pageDictionary, "Annots", &outputArray) where pageDictionary is the CGPDFDictionary representing the CGPDFPage, and outputArray is the variable (CGPDFArrayRef) to store the Annots array of that page in.