How do I convert a PDF stored as NSData to a CGPDFPageRef?

Please see 开发者_开发技巧the following code:

NSString *pdfPath = [documentsDirectory stringByAppendingPathComponent:@"test.pdf"]; 
NSData *myData = [NSData dataWithContentsOfFile:pdfPath];  

How do I convert myData to a CGPDFPageRef?

Why not use CGPDFDocumentCreateWithURL?