var doc = PdfDocument.Load(@"d:\0\9\mp4.pdf");
var pageDict = doc.Pages[0].Dictionary;
FS_RECTF cropBox = ConvertToPdfRect(pageDict["CropBox"] as PdfTypeArray);
FS_RECTF bleedBox = ConvertToPdfRect(pageDict["BleedBox"] as PdfTypeArray);
FS_RECTF trimBox = ConvertToPdfRect(pageDict["TrimBox"] as PdfTypeArray);


private static FS_RECTF ConvertToPdfRect(PdfTypeArray cropBoxArray)
{
    return new FS_RECTF()
    {
        left = (cropBoxArray[0] as PdfTypeNumber).FloatValue,
        bottom = (cropBoxArray[1] as PdfTypeNumber).FloatValue,
        right = (cropBoxArray[2] as PdfTypeNumber).FloatValue,
        top = (cropBoxArray[3] as PdfTypeNumber).FloatValue
    };
}