Crop pdf file

When you say "crop" what do you mean? Do you mean you want to delete some of the pages, or do you want to crop part of a single page?

In general, you have very limited options on the iPad itself (as opposed to transferring the PDF file to your laptop and then using any one of several applications that can do either or both of the above functions).

There is, however, one possibility on the iPad, depending on how much of a given page you want to crop. You could use the screen capture feature of the iPad (simultaneously press the Home button and the On/Off button on top) to save the screen as a photo and then use a photo-editing app to crop it as you wish.