To crop a page in a PDF document.

doc = PDFDoc.new(filename)

# Access a PDF page
page = doc.GetPage(1)

# Crop the page
page.SetCropBox(Rect.new(0, 0, 500, 600))

The crop box defines the region to which the contents of the page are to be clipped (cropped) when displayed or printed. Unlike other types of boxes, the crop box has no defined meaning in terms of physical page geometry or intended use; it merely imposes clipping on the page contents. The default value is the page's media box. A new crop box can be imposed on a page with Page.SetCropBox().