How can I crop a PDF on iPhone/iPad?
iBooks does not allow you to crop PDFs. How can I crop a PDF on my iPhone or iPad?
iPhone 6s Plus, iOS 11.2.5
You likely need to purchase an app to do this.
One such app is PDFPen. I believe this app can crop PDF documents, but am not 100% certain.
PDFpen 3 – PDF Editor on the App Store
(I have no financial interest in PDFPen. I am a former customer of PDFPen on the Mac.)