Please provide the exact dot version of the software & OS installed at your system .Also check whether is there any update available for the software after going through "help > check for updates "
Would Request you to please refer to this KB Article :- https://helpx.adobe.com/acrobat/using/crop-pdf-pages.html
If issue still persist, would request you to please Recreate the document using Adobe PDF Printer or Print as image option & then check with the same.
I hope you are following the correct steps to crop the pages and change the margin setting.
Crop Pages>Select the area you need to crop>Once selected you would see the area is highlighted by a box>Double click on the box>Next dialog box would give you various options where you would see the margin dimension and would be able to customize it.
I guess you did not try that.
This is what you would see.
Cruciallly there are no margins in a PDF. None. Nothing at all. What you see as a margin is just a bit of the page which happens to have nothing in it. Acrobat puts things where the PDF tells it. In this way it is utterly unlike Word, which will move stuff around using what you tell it are margins.
So you can crop pages, but nothing will tell you what the "margins" are because they are just an accident of where things are sitting. I hope this has made things clearer, but I fear it may not have.
Thank you. I took forever to find the version info in adobe. Its
2015.006.30119. I'm using windows 10 home edition OS build 10586.71
None of the directions I've tried online have worked...