Hi Richard, place the large PDF onto a Pages page, then do a screen shot (shift-cmd-4) this will create another PDF that may be smaller, repeat this again and each screenshot will be smaller. I have just reduce the size of a PDF from 9MB to 668kb in about 4 of goes.
PDF is designed as a high fidelity output format, it's not really something you should be trying to edit. The size of a PDF is generally driven by bit image graphics (eg JPEGs) inside it so if you have the original document that generated the PDF it is quite easy to attack the quality and hence size of these.
But if you need to get there and really must start from here: there are some apps that can reconstitute a reasonable version of the original, giving you the chance to reduce the size of any bit images. Adobe's Acrobat is the gold standard but my current favourite is Affinity's Designer. Publisher will likely be even better when finished but the Beta version is currently free and might help (it can open my multi-page test PDF).
Watch out for any errors/omissions created by importing the PDF and don't expect it to be a trivial task.