Cropping would be my preferred way of reducing a large page to just a smaller portion of that page. You can access the crop tool by e.g. right-clicking on the page in the page thumbnails navigation pane on the left, and then select "Crop Pages...", or by selecting the crop function from the "Organize Pages" toolset in Acrobat DC.
The only potential downside of cropping is that the original page content is still available, which means that first of all, the document size will not change, but also, that somebody with access to the crop tool can restore the original content.
Thanks for you help. That's the problem - the cropped image is then so small it can't be read. Ideally for scaling off it is best at 100% but then you can't crop the area as it's bigger than the screen?
If it's a vector drawing, you should be able to zoom in, and if that is possible, you can set the initial zoom value so that whenever you open the document, it will be zoomed to that setting. You can do that via the document properties (Ctrl-D or Cmd-D), then go got the "Initial View" tab and set the "Magnification" to your desired zoom setting. You can either pick a selection from the list, or type in your zoom factor.