PDF is a document format, so that would be like opening a Word file and "Cropping" a page. So the question to ask is what you are looking to do? Are you wanting to crop an image within the PDF page (keeping the page format, e.g. A4), or are you looking to change the page format (e.g. crop an A4 page to A5 for example).
Of course, you can load a PDF as if it were an image, crop it and then save it back as a PDF (as an image within a custom format PDF page), but that is not really what PDF is for.
Sorry for a very late reply. Actually I was out of office traveling.
What I am actually looking for is to crop the PDF (which is generally a one page PDF) rearrange the elements that there after cropping and save it as a PDF. The saved PDF file's page size would naturally change to the cropped size.
The functionality that I want is available in an online service.