cropping pdfs does not seem to work with certain pdfs

Hi all,

I am trying to crop a pdf which does not seem to work. It works fine with most pdfs but it does not seem to work with pdfs that I got from converting a webpage through web2pdfconverter.com . On these pdfs the cropping option simply does not appear

anyone a suggestion how to get webpages into pdfs and still crop them?

thanks,
patrick