public void compare() 
	{
		// already found differences (e.g. different page count)
		if(!pdfInfoHolder.isDifferent())
		{
			PDFHolder pdfHolder1 = pdfInfoHolder.getPDFStructure1();
			PDFHolder pdfHolder2 = pdfInfoHolder.getPDFStructure2();
	
			// missing the counter part
			if(pdfHolder1 == null || pdfHolder2 == null)
			{
				pdfInfoHolder.setDifferent( DifferenceType.VISUAL);
				return;
			}
			
			//compare the structures, in both directions
			comparePDFHolder(pdfHolder1, pdfHolder2);			
			comparePDFHolder(pdfHolder2, pdfHolder1);
			
			// check if a difference was found on one of the pages.
			// if so mark the entire pdf as different
			pdfInfoHolder.checkDifference();
		}
	}