Render UTF8 PDF correctly using CGContextDrawPDFPage

Question

I have a japanese PDF document that I'm trying to render correctly on the iPad.

I'm using the following code:

CFURLRef pdfURL = CFURLCreateWithFileSystemPath(kCFAllocatorDefault, (CFStringRef)pdfFilename, kCFURLPOSIXPathStyle, false);
CGPDFDocumentRef pdfRef = CGPDFDocumentCreateWithURL((CFURLRef)pdfURL);
CFRelease(pdfURL);

CGPDFPageRef pdfPage = CGPDFDocumentGetPage(pdfRef, page);

CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
CGContextRef context = CGBitmapContextCreate(NULL, 
                                             pdfPageSize.size.width*pdfScale,
                                             pdfPageSize.size.height*pdfScale,
                                             8,
                                             (int)pdfPageSize.size.width*pdfScale * 4,
                                             colorSpace, 
                                             kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);
CGColorSpaceRelease(colorSpace);

CGPDFPageRetain(pdfPage);
CGAffineTransform transform = aspectFit(CGPDFPageGetBoxRect(pdfPage, kCGPDFBleedBox),
                                       CGContextGetClipBoundingBox(context));
CGContextConcatCTM(context, transform);
CGContextDrawPDFPage(context, pdfPage);
CGPDFPageRelease (pdfPage);

CGImageRef image = CGBitmapContextCreateImage(context);
CGContextRelease(context);

The japanese characters are not showing correctly (i.e. boxes instead of the character). I am not aware of any character set parameter which could configure the pdf handling.

Does anybody know how to correctly handle foreign characters?

Thank you all in advance!